Make media openable with the openLinks keybind

This involved a refactoring to move all the media handling functions
(downloading, opening externally, etc) out of the Event delegates and
into the EventList, which manage keybinds instead.

This should also be better for performance since all these functions are
no longer duplicated for every Event in view.

Other user-noticable change: clicking on non-image media will
always download and open them no matter if the room is encrypted or not,
instead of opening non-encrypted files in browser like before. It will
be possible to still do that with an "open externally" command later.

TODO.md

@@ -1,10 +1,15 @@
 # TODO
 
 - Image viewer: 
+    - fix gifs
+    - double click on img to open fullscreen
     - open externally in context menu in timeline thumbnail
     - hflickable support kinetic scrolling disabler and speed/decel settings
     - buttons 
     - keyboard controls
+    - prevent drag-scrolling timeline when image opened
+    - eventfile middle click
+    - right click in dark space of image viewer
 
 - clipboard preview doesn't update when copied image changes until second time
 - Avatar tooltip can get displayed in front of presence menu