- Neatly placed all emerge related code into a new file, emerge.cpp
- Greatly cleaned up the code in EmergeThread::Thread()
- Reworked Emerge queue.  Now an actual std::queue of v3s16 block positions
- Removed the completely unnecessary map of peer ids requesting blocks

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.