Add DnD Quick Toggle (and some style changes)

[Leleat]

This PR adds a Do-no-Disturb Quick Toggle and moves the clear-notifications-button to the Notifications title (similiar to Libadwaita's header suffixes). Here is how it looks with GNOME Shell's default theme (on Fedora)

(The clear-notifications-button has a slight highlight when hovered.)

Some notes:

• Should the separate notifications popup be removed entirely? It currently doesn't look very good with the styling.
• I've renamed the 'button remover' file and references to 'quickTogglesManager' because now it doesn't 'only' remove existing quick toggles. In the future I imagine there could be more quick toggles (like a11y etc) implemented by this extension or even the ability to reorder the quick toggles.
• Since you wanted to refactor the code anyway (Code refactoring, and improve quality of code needed #16) I've mostly tried to build on top of the existing code.