macosx: Add asset catalog with accent color, respect system accent color settings for accented views

This MR adds an asset collection with a VLCAccentColor colorset used as the main accent colour for the application. This both lets us more easily add our accent color to more views (including menus in the application) while also respecting the system accent color (on macOS 10.14+)

Note that there are still elements force-coloured orange in parts of the app (i.e. because the image is orange, in the case of some buttons for instance)

Examples:

Screen_Recording_2022-12-31_at_12.06.27 Screen_Recording_2022-12-31_at_12.06.56

Signed-off-by: Claudio Cambra developer@claudiocambra.com