Add background and text colour options to the widgets

[PyFlat-JR]

What does this PR do?

Adds background and text colour options to both widgets.

• The Appearance tab gets Macro Deck's background colour (including Transparent) and label colour fields. Both widget types declare BackgroundColor and LabelColor as appearance properties, so the Set Background Color and Set Label Color actions reach them too.
• Background: passed to the widget's root as stored. A transparent root also drops the deck's own tile face, so the folder background shows through.
• Text colour: the percentage uses the colour as is. Names and captions use it fainter so the widget keeps its hierarchy. Text colours only take #rrggbb (an 8-digit hex is ignored), so the fainter shades are mixed toward an opaque background (80 % and 60 %); over the theme's face or a transparent background every text uses the full colour.
• The ring's empty track keeps the theme colour: UiGauge has no track colour (requested in [SDK] Track colour for UiGauge Macro-Deck-App/Macro-Deck#1251).
• New previews: "Tile - custom colours", "Tile - transparent background", "Panel - list with custom colours".

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 252 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a device backend: I tested it against the real hardware (see below).

Device/hardware tested against (if applicable)

Not a device backend.