Plasma6 initial support

[sendevent]

Plasma 6 Port

This PR ports the plasmoid to P6, modernizing its components and improving usability. It also provides some fixes/UI refinements/code cleanups.

@MarcinOrlowski I'm not sure about your plans or progress on porting, or whether you want to keep P5 and P6 separate, or drop P5 and focus solely on P6. However, this PR could help with whatever versioning policy you prefer and is relevant to #93 and #99.

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Plasma 6

• Actualized metadata
• Updated QML imports and removed versioning from statements.
• Replaced deprecated components with their P6 counterparts.

🎨 UI/UX Changes

📜 Context Menu

• Listed alternative plasmoids family.
• Added actions to access System Settings directly from the plasmoid context menu.
  

ℹ️ About

• Replaced the custom AboutDialog with Plasma’s standard AboutPlugin.
• Added a "Report a bug..." button to the "About" section.
• Listed contributors.
  

🛠️ Miscellaneous Improvements

• Corrected metadata.json to align with best practices.
• Adwanced .gitignore to exclude QtCreator and other files.
• Replaced CalendarView with its upgraded version from org.kde.plasma.digitalclock.
  The new version keeps the same style but is a bit less compact and includes a dedicated button for quick access to the plasmoid's configuration.

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🖥️ Development & Testing

I verified that the clock and calendar work both as a standalone widget and as an item in the system tray. The Config UI still needs some work, and I haven’t extensively tested other parts beyond what was needed for my digital-clock CSS:

Tested on:

• OS: Debian GNU/Linux 12
• KDE Plasma Version: 6.2.5
• KDE Frameworks Version: 6.10.0
• Qt Version: 6.7.2
• Kernel Version: 6.12.12-amd64
• Graphics Platform: Wayland

[MarcinOrlowski]

Thanks a lot for your contribution! Unfortunately I had no time to push these works myself so this looks like a X-Mas for me :) Please give me a moment to review the changes you made.

As for Plasma 5/6 - support I plan to create v5 or plasma5 branch at current HEAD, so in case of any changes needed to be for older version it should not be a problem. And I do not plan to drop Plasma 5 as of yet.