Implement message spoilers

[diamondburned]

Spoilers aren't implemented in gtkcord4 yet, even though they were implemented in gtkcord3.

There are 2 main approaches:

1. Attach an EventController to check for mouse hovering events, if the mouse is hovering over the spoiled text span then reveal it, or
2. Add a button that reveals the text when pressed.

1 is a lot harder since I've yet to figure out the right API to use for this. 2 is a lot easier, but it's a lot less convenient too.