Add alphabeticaly sorted worldlists
Select world after creation
Move worldlist handling to separate file
Merge world selection of singleplayer/server tab
Remove some useless code

This reverts commit 39d03148.

Always use address for server
Fix first (empty) element beeing selected after deleting last world

make enter button work in create world dialog

Do a check for valid num_files before doing
assert(!m_mesh_update_thread.IsRunning());

In particular, don't execute the assertion if all media has already
been received.

Also write something about rebuilding shaders to infostream in
Client::afterContentReceived()

Fixes a denial of service vulnerability, issue #784