Fix crash when no world is selected and configure button is pressed.

by moving return statement out of if-then-else clause...
Improve behaviour for empty modpacks and when no mods at all are installed:

Only show enable all / disable all buttons for all add-ons when at
least one add-on is installed. When no add-on ist installed, don't
show any buttons or checkboxes.

Added is_modpack flag to ModSpec to distinguish empty modpacks from
normal mods and check this flag in mod selection gui so that empty
modpacks are not treated like mods that can be enabled or disabled.
Fix crash when pressing delete button in server browser and no server is selected.

A check for that was there, but was comparing an unsigned variable to
-1, which doesn't work.
Make sure that settings are written to config file when settings are removed.

Previously, settings where only written when a value has changed, and
removal of a setting value didn't count as a change.
Update translation files
Merge pull request #436 from doserj/mod_selection

Fix some crashes and improved behaviour for mod selection gui
Treegen update. Some new symbols. Speed up code a bit.
Tweak buildbot

* CURL support
* only download packages if they don't exist
* fixed download links (libvorbis and libogg)
* manual downloading if automatic downloading fails (only minetest and minetest_game)
* mingwm10.dll and openal_stripped.zip are downloaded automatically
Make mapgen factory setup more elegant, add mapgen_v6.h
Fix compile under MingW
Slightly improved version of mystrtok_r
Translated using Weblate (German)
Merge remote branch 'origin/master'
check parameters for minetest.log lua function
Merge pull request #447 from sapier/add_lua_log_parameter_check

BUGFIX: check parameters for minetest.log lua function
Workaround failing Travis clang build.
Place block when holding sneak while right-clicking nodes with formspec
Dont call on_rightclick() if sneak is pressed
Add Spanish, Russian, Polish and Romanian languages.
Merge pull request #441 from kwolekr/mapgen_factories

Make mapgen factory setup more elegant, add mapgen_v6.h