Introduce a StyleManager

[leolost2605]

This is pretty much #1857 and therefore closes #1857

Only a bit refactored and it will now also be used to get the color scheme. This is because currently granite is used to get the color scheme which in turn uses the portal. This caused start time on wayland to be > 20 seconds since we are waiting for the portal. This PR reduces start time to the same as on X so about 1 - 2 seconds.

Fixes #1674

Continues towards #1839 (AFAIK only shadow effect is now still using GTK)

[leolost2605]

My laptop just ran out of battery lol 🥲
I still need to do a push then this should be ready

[leolost2605]

We could now also drop Granite as a dependency for the gala lib and main gala executable however that would be an API break since Drawing.Color implements Granite SettingsSerializable. Not sure how we handle gala API? Is it supposed to be stable?

[danirabbit]

I think the only thing using Gala API is other Pantheon components so should be okay to break imo 🤷🏻‍♀️

[leolost2605]

Ok I dropped the granite dependency in libgala and gala but I left the methods that are implementing settingsserializable in Drawing.Color so that plugins that in theory exist and need it are only a line change and recompile away from getting it working again. Dropping these can probably be done in follow up PRs IMHO.

[danirabbit]

Works as expected and we have tintou's ack on the other PR with his suggestion to use accountsservice so I'm happy with that 🚀