Allow derived bars to provide a passive background component

[adamritter]

Derived bar plugins currently need to duplicate Bar.qml to change how its background is painted. Add an optional backgroundComponent so they can inherit the stock bar's widgets, layout, input handling and multi-monitor behavior while supplying their own passive background.

Each monitor gets its own disabled Loader beneath the widgets, with the bar's background color, position and orientation exposed as context properties. Null keeps the existing theme fill, and transparent mode unloads the custom background. No appearance change for existing bars. Includes a usage example and focused QML tests.

Validation:

• Existing bar tests and new Qt 6 background tests pass (default/removal, geometry/theme/orientation, independent instances, disabled input and transparency).
• Verified the modified Bar.qml through a derived plugin in the running desktop shell: gradient background, transparency, and return to the default fill. The local shell predates this branch, so the current PluginBarApi was included in the temporary test plugin.
• Ran ./test/all: six shell test files fail on this machine (config, launch-about, locate, network-qr, snapper, unowned-system-paths). Re-ran each against the unmodified base and reproduced all six failures; some require the separate packaging checkout. Relevant bar tests pass.

Visual verification

Actual desktop captures of the same derived bar with the stock clock, on an empty workspace. Cropped to the bar and wallpaper immediately below it.

Custom gradient supplied through backgroundComponent

Transparent mode: custom background unloaded

Component removed: stock theme fill restored