Previously, testing in macOS required running `make install`, switching
over to the Finder, then double clicking TuxPaint.app, which was a time
consuming process. This change allows the tuxpaint binary to be run
directly from the command line (./tuxpaint) without `make install`, which
makes it easier to test any small code change. Now `make install` is only
required to build the TuxPaint.app bundle for execution from the
Finder.
FYI, `make install` also builds TuxPaint.dmg for distribution. A small
change is included in this commit to allow TuxPaint.dmg to be built without
building TuxPaint.app (if, for example, you delete TuxPaint.dmg by accident
and need to rebuild it.) This is done via `make TuxPaint.dmg`.
