Tools to create nice signed DMG

These are only useful for me, but having them in git makes my life so much
easier...

Instead of using macdeployqt to create my DMG I use the tool that I used for
Subsurface 3. This allows for much prettier DMG content as well.

Fixes #329

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>