Previously, both the .desktop.in template and the final .desktop file
were checked into source control.
While in some ways convenient, the manual step of updating it may be
forgotten after updating a translation or adjusting the template. It
also potentially introduces confusion as to which file to modify.
Instead, generate the .desktop file at build time. Since this is now
done using msgfmt rather than intltool, there is no additional
dependency beyond gettext, which is already used at build time to
compile .po files to .mo files.
Previously `make install` on macOS created TuxPaint.dmg, but it was a
simple disk image.
Now TuxPaint.dmg is formatted as one would expect from any other macOS app,
with a Tux folder icon, symlink to the /Applications folder, and a
background image with an arrow.
