Add ColorManager for theme accent color

[leolost2605]

• Gets the theme accent color from settings portal
• Add a ColorManager because we are going to need it for more things like background color light and dark, tooltip color etc. since we currently use gtk.stylecontext for these colors and this is problematic (see Remove the usage of GTK inside the compositor #1839)

Fixes #1674

First step towards #1839

[tintou]

While I agree with the implementation, I don't think that we should use the portal as the portal will be available after Gala. For this I think that we should use the elementary AccountService extension

[leolost2605]

I updated it to use the AccountsService :)