This is the first implementation of a theme manager. It’s a rebased and
revisited version of #792.

Now uses the same as in the deck editor

Previously all columns were the same width, making it cramped. I have
resized them to give a better initial width.

Hide filters-related buttons
Bypass the proxy model

load settings from saved values or use defaults.

Remove some duplicated code.

Can now join a room by highlighting a room and pressing ENTER. Works
nice with arrow navigation.

Can now double click on a room in the lobby to join

context menu for a message sender's name in chat; also display the user level icon next to the name; minor consistency and type-safety changes

IMO these should not have been split

moderators can override game restrictions and kick players when spectating; game host privileges shift when host leaves

changed chat indentation, allow leftclick for CardInfoWidget in ChatView, changed GameSelector indentation