Change large amount of objects to be configurable via a setting in minetest.conf (max_objects_per_block).

Add old style mod selection in worldconfig dialog

Two new configuration options are added:
     - "enable_ipv6" to enable/disable the overall use of IPv6
     - "ipv6_server" to enable/disable the use of IPv6 sockets when running
       a server (when "enable_ipv6" is enabled)

Now Environment::clearAllObjects() unloads unused blocks in an interval
defined by max_clearobjects_extra_loaded_blocks (default 4096).

Also add a menuheader, menusplash, menufooter_clouds, and allow HD menu footers.
And finally don't git-ignore textures/base/.