Notifications overhaul

[Discookie]

• Added a sidebar view to show past notifications
• The notif's actions can still be executed through the sidebar, even if the user disabled pop-up notifications
• Ported all existing notifications to the new system, but did not add any new ones yet

[Discookie]

Refactored the notifications system, to support updating previously-added notifications.
In future commits, this will be used to display the status of a running CodeChecker process in the more detailed way than it's currently shown, and I want to avoid the need to review it twice.

[vodorok]

Looks good to me, but I need to test it before merging.
Do you have ideas to test this feature?

[vodorok]

LGTM