Introduce BigScreen styled FullScreenOverlay & update Favourites Manager style

Seshan Ravikumarrequested to merge
seshpenguin/plasma-bigscreen:seshpenguin/fullscreenoverlay-work into master

This MR introduces a new FullScreenOverlay component, based on the NanoShell.FullScreenOverlay. It is a styled full screen shell overlay component with a header, close button and animation. It is intended to be shared across similar full screen overlays within the shell.

The first usage of this overlay is in the Favourites Manager, which has been slightly modified visually and now includes add and remove icons.

Favourites Manager Before: Screenshot_20250324_233134

Favourites Manager After: Screenshot_20250325_233454

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