Allow filtering windows shown as workspace app icons

[atimofeev]

Problem

With Show Workspace Apps enabled, workspace indicators display an icon for every window reported by the compositor. Some utility, helper, picture-in-picture, or otherwise non-representative windows should not appear, but disabling showWorkspaceApps removes all application icons.

Niri window rules cannot solve this cleanly: they control compositor behavior, but do not mark a window as hidden only from DMS. DMS owns this rendering in WorkspaceSwitcher.qml.

Proposed behavior

Add optional exclusion rules for workspace application icons:

• Match by application ID/window class.
• Optionally match by title, allowing one window type from an application to be hidden without hiding every window from that application.
• Apply exclusions before grouping and icon-count calculation, so layout and maxWorkspaceIcons remain correct.
• Keep current behavior when exclusion list is empty.

Possible settings UI: Workspaces → Workspace Apps → Excluded Windows, using application ID/class plus optional title pattern.

Exact setting shape and matcher syntax are open for discussion. Reusing DMS's existing normalized app ID handling would keep matching consistent with current workspace icons.

Alternatives considered

• Disable Show Workspace Apps: hides every icon, too broad.
• Niri window-rule with exclude: exclude only prevents another Niri rule from matching; it does not exclude a window from DMS or compositor IPC.