Option to change notification click action

[freaktechnik]

Currently notifications always open a new tab with the stream. Should have similar configuration options to the popup.

See #314 . Also kind of intertwined with #233

[freaktechnik]

This should actually be done with an editor similar to actions #433 , where you can instead choose the action to perform on notification click. All actions that apply to the notification conditions should be available.

Notifications have a field that has to change in order for them to trigger, the rest is pretty similar to actions. Further notifications should be toggleable, so you don't have to delete them when you don't want to see them.