Add a list of servers to the "Multiplayer" tab

If USE_CURL is set, it also downloads a list from a remote server.
The url of this list is configurable in minetest.conf using the setting "serverlist_url"
The local list of favorite servers is saved in client/serverlist/filename
filename is also configureable using the setting "serverlist_file"
Basic support for configuring which mods to load for each world

settings.h: added function to return all keys used in settings, and a
function to remove a setting

mods.{h,cpp}: added class ModConfiguration that represents a subset of the installed mods.

server.{h,cpp}: server does not load add-on mods that are disabled in
the world.mt file. mods are disabled by a setting of the form
"load_mod_<modname> = false". if no load_mod_<modname> = ... setting
is found, the mod is loaded anyways for backwards compatibilty. server
also complains to errorstream about mods with unstatisfied
dependencies and about mods that are not installed.

guiConfigureWorld.{h,cpp}: shows a treeview of installed add-on mods
and modpacks with little icons in front of their name indicating their
status: a checkmark for enabled mods, a cross for disabled mods, a
question mark for "new" mods

Mods can be enabled/disabled by a checkbox. Mods also show a list of
dependencies and reverse dependencies. double-click on a mod in
dependency or reverse dependency listbox selects the corresponding
mod. Enabling a mod also enables all its dependencies. Disabling a mod
also disables all its reverse dependencies.

For modpacks, show buttons to enable/disable all mods (recursively,
including their dependencies) in it.

Button "Save" saves the current settings to the world.mt file and
returns to the main menu. Button "Cancel" returns to main menu without
saving.

basic keyboard controls (if the proper widget has keyboard focus):

up/down: scroll through tree of mods
left/right: collaps/expand a modpack
space: enable/disable the selected mod