(style): add service and preference to switch from one theme to another

What does this MR do?

Add a service to switch style/theme live using preferences.

Selected style is saved in preference file (if it exists), and then reloaded when you re-launch the app.

How to test it?

1. Compile ExNotifications or even VRRender
2. Run it!
3. change the theme color preferences
   • ExNotifications: last button of the right
   • VRRender: preferences menu.
4. Enjoy.
5. Close
6. Re-launch.
7. Previous theme should be applied.

Data

No data required.

Some results

Additional tests to run

• Build on Windows
• Build on Linux

Associated Issues/Merge Requests

Closes #449 (closed)