Introduce a style manager

[leolost2605]

This moves the provider management etc. from the init function to a separate class StyleManager.

This class now also automatically handles adjusting the theme to system preference and allowing the application to override this (which was my motivation since that's a bunch of boiler plate code that then could be removed from the applications startup functions).

Since more stuff is going to have to be done here (see e.g. https://gitlab.gnome.org/GNOME/gtk/-/issues/6821) this might be a good way to go now.

This currently is a global style manager for all displays but maybe we should follow Adw.StyleManager and make it per display? This is now per display since that makes more sense in general

[leolost2605]

Regarding the demo: I opted to leave the switch that directly interfaces with gtk settings in the headerbar which can cause a mismatch between e.g. force dark in the stylemanager page and a light mode of the app. This is intended behavior because we shouldn't actually do anything with gtk settings manually but it might confuse people who use the demo.

[leolost2605]

Fyi I switched it to per display style managers because that just makes more sense and is a lot nicer in code (no more need for lots of foreachs)

[danirabbit]

Hm so testing this, it doesn't seem to work in the demo and I get:
** (granite-7-demo:140614): CRITICAL **: 11:57:41.997: granite_style_manager_set_color_scheme_override: assertion 'self != NULL' failed

[danirabbit]

we shouldn't actually do anything with gtk settings manually but it might confuse people who use the demo.

I think we should probably remove the switch to make sure the demo is working properly and contains only good example code and not stuff we don't want other people to copy

[leolost2605]

Hm so testing this, it doesn't seem to work in the demo and I get: ** (granite-7-demo:140614): CRITICAL **: 11:57:41.997: granite_style_manager_set_color_scheme_override: assertion 'self != NULL' failed

This should be fixed now, for some reason static construction didn't seem to work?

[danirabbit]

Works as expected, looks good, just a few comments!

[danirabbit]

@alice-mkh Any thoughts on how this will/won't interact with Adw in for example Epiphany?