Feat: notification.add_action

[evnchn]

Motivation

Addresses NiceGUI Wishlist (view), how we had to leverage kwargs to define actions for notifications.

Implementation

• I strongly disagree with the original proposed ui.notification_action, since:
  • It is not a UI element
  • We can't leverage the expansive .on method, which covers both Python handler and JavaScript handler
• Therefore, I chose for notification.add_action, which lets adding a button (yep the action is a QBtn definition, check Quasar docs) which triggers an event when clicked
• Then, we can leverage .on to listen to the event we just defined.
• To enable Quasar, Tailwind and CSS colors, I stole the logic from mixins.color_elements, but now since it is not an element but just a dictionary, we update it a bit differently.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

-> Do we need pytest? If so, tell me and I'll take a shot at it...