Add a counter to AdwViewSwitcher

This is the Merge Request for the counter property. This will close #203 (closed).

Checklist for technical points:

• AdwViewStackPage does not implement GtkAccessible, which leads to errors.
• Fix the size of the badge.
• The vapi-generator complains about the add-child function and a wrong return type, resulting in the CI failing.
• Clean up stylesheet
• Other problems I've not found yet

Checklist for design points:

• Decide style for "non-urgent" counter
• Decide if we need separate style for checked button
• Decide where the counter can be placed when ViewSwitcher is large
• Provide a HighContrast variant of the badge

Images of the current state: