Use palettes for most theming

Can't do it for QtWidgets code. We need to fully transition, before we
can fully switch over probably.

Update Ubuntu 19.10 dependencies list

Removed double-sanitization of names in "... is typing" messages

fix #155: use correct subsystem on windows

Better image overlay handling when downloading

 - hides the overlay when prompting for download location
 - cancel re-shows the dialog
 - success closes the overlay
 - would be nice to have a return code from the download fn in
 mtxclient.

Closes #140

Place unread rooms higher in the room list

Fixed bug caused by me forgetting what types im using (invites appearing at the bottom of the list instead of the top when priority sorting was off)