Remerge some files in order to reduce number of additional files

Make necessary changes for split, rename files, reorganize some bits

- 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.