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

Buddies will now be seen as stars in the chat

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

added option to ignore unregistered users in chat; switched UserList data storage to QMap to speed up lookups; issue #28 fixed

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

fix: don't show conceded players, fix: compute multicolumn layout correctly when dealing with odd number of players