Improve Windows text input for international keyboard layouts and IMEs

[reflectronic]

• Custom handling of dead keys has been removed. UX for dead keys is now the same as other applications on Windows.
  • We could bring back some kind of custom UI, but only if UX is fully compatible with expected Windows behavior (e.g. ability to move the cursor after typing a dead key).
  • Fixes Double quotes can only be followed by enter or space (German/Belgian/French keyboard) #38838
• Character input via AltGr shift state now always has priority over keybindings. This applies regardless of whether the keystroke used the AltGr key or Ctrl+Alt to enter the shift state.
  • In particular, we use the following heuristic to determine whether a keystroke should trigger character input first or trigger keybindings first:
    • If the keystroke does not have any of Ctrl/Alt/Win down, trigger keybindings first.
    • Otherwise, determine the character that would be entered by the keystroke. If it is a control character, or no character at all, trigger keybindings first.
    • Otherwise, the keystroke has any of Ctrl/Alt/Win down and generates a printable character. Compare this character against the character that would be generated if the keystroke had none of Ctrl/Alt/Win down:
      • If the character is the same, the modifiers are not significant; trigger keybindings first.
      • If there is no active input handler, or the active input handler indicates that it isn't accepting text input (e.g. when an operator is pending in Vim mode), character entry is not useful; trigger keybindings first.
      • Otherwise, assume the modifiers enable access to an otherwise difficult-to-enter key; trigger character entry first.
  • Fixes Can't insert curly brace with German keyboard layout on Windows #35862
  • Fixes Windows Beta: Keymaps do not handled correctly on Windows when using AltGr (Again) #40054 (comment)
  • Fixes Windows: AltGr not working on Croatian keyboard #41486
• TranslateMessage calls are no longer skipped for unhandled keystrokes. This fixes language input keys on Japanese and Korean keyboards (and surely other cases as well).
  • To avoid any other missing-TranslateMessage headaches in the future, the message loop has been rewritten in a "traditional" Win32 style, where accelerators are handled in the message loop and TranslateMessage is called in the intended manner.
  • Fixes Windows: Hangul toggle key not working #39971
  • Fixes Windows: Half-width Japanese characters don't work, can't use IME in agent panel #40300
  • Fixes Windows: Japanese IME (ATOK) input toggle keys not working properly on Windows #40321
  • Fixes Windows Beta: Language input keys does not work in the Korean Microsoft IME. #40335
  • Fixes Windows: Can't change Microsoft IME to Japanese input mode when I push "Zenkaku/Hankaku" key #40592
  • Fixes Windows: Cannot change Google Japanese Input IME on/off with Eisu key #40638
  • As a bonus, Alt+Numpad entry now works, since it is handled by TranslateMessage.
    • One (minor?) issue is that we don't differentiate between numpad and alphanumeric input. Thus, when NumLock is on, Alt+Numpad gets interpreted as our tab-switching keybinding, and won't trigger Alt+Numpad entry. This doesn't seem so difficult to fix later if needed.
    • Fixes Windows: can't use alt+numpad code to type #40699
  • As a bonus, Alt+Space now opens the system menu, since it is triggered by the WM_SYSCHAR generated by TranslateMessage.
• VK_PROCESSKEYs are now ignored rather than being unwrapped and matched against keybindings. This ensures that IMEs will reliably receieve keystrokes that they express interest in. This matches the behavior of native Windows applications.
  • Fixes Windows Alpha: IME backspace will be Intercepted by editor #36736
  • Fixes Windows Beta: Chinese IME preedit text cleanup issue causes confirmed characters to be deleted on backspace #39608
  • Fixes Windows Beta: IME bug when press Tab key to accept an AI code suggestion #39991
  • Fixes Windows: KeyUp event is not send to IME #41223

Release Notes:

• windows: Improved keyboard input handling for international keyboard layouts and IMEs