i.e., not only the mod with missing dependencies, but also the missing
dependencies itself. (This already used to be the case before the mod
selection gui was added)

Also, mods with unsatisfied dependencies are no longer reported as
mods that could not be found.

This way, plants actually show the real backface on their back side,
i.e., the front face mirrored around the vertical axis, instead of
showing the front face on both sides. This looked weird when the
texture was not symmetrical around the vertical axis.

All uses of operator[] in guiConfigureWorld.cpp have been replaced
with .find() operations.

In some places, operator[] had been called with mod names of "Add-Ons"
or "N/A", which would insert a default-constructed ModSpec into the
list of add-on mods. In other places, the use of operator[] was safe
now, but it probably is safer for future changes to use .find()
everywhere.

Previously, settings where only written when a value has changed, and
removal of a setting value didn't count as a change.

A check for that was there, but was comparing an unsigned variable to
-1, which doesn't work.

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.

by moving return statement out of if-then-else clause...