File reorg: Makefile: Put each filename on seperate line

This will make the diffs clearer in the upcoming code reshuffle.
python packagisation: Rename stbt to stbt.sh, but only in source dir

Rename the "stbt" executable (and the "stbt.in" it's generated from) to
"stbt.sh" (and "stbt.sh.in"). But still install "stbt.sh" as
"$prefix/bin/stbt".

We need to rename it in the source directory so that it doesn't clash
with the python package (added in the next commit) that is a directory
called "stbt".
python packagisation: Move stbt.py to stbt/__init__.py

This is the first step in converting stbt from a python module into a
python package. I've only moved stbt.py to stbt/__init__.py; to make
rebasing easier I'll make any necessary changes in separate commits.
python packagisation: Fix relative paths in stbt/__init__.py

In the previous commit I moved stbt.py (unchanged) to stbt/__init__.py.
Now I'm fixing the relative paths affected by the move.
python packagisation: Move gst_hacks.py into stbt package
python packagisation: Move pylint plugin into stbt package