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.