ubuntu: Add support to selecting a theme accent color

add assets for colors:

- add colors under assets/colors.
- update theme-thumbnails to look similiar to GNOME-42 ones.
- add custom-style classes for colors and update remaining style-classes to match new theme-thumbnails.
- update ubuntu.gresource.xml and meson.build to include new color-svgs.

add colors to .ui file:

- Mimic the look of GNOME-42 Appearance-panel for theme-switcher by use of HdyPreferencesRow, Hd>
- Bring out GtkLabel(s) from inside of GtkFlowBox with use of GtkGrid.
- Rename Window-colors to Styles.
- Add Colors-flowbox below theme-flowbox by use of HdyActionRow.
- make use of custom-styles colors, colors-thumbnails for colors.

Add colors to ubuntu-panel.c:

- Use GtkFlowBox and GtkFlowBoxChild for colors.