Remove console window from standalone

[hoechenberger]

Currently a console window pops up when a user starts the standalone .exe. This is good because closing that window is the only way to quit the backend. But it's ugly and could irritate users.

We should

• get rid of that window
• add a "shutdown pyqmix backend" (or similar) button to the frontend
• check whether a backend process is already running, and if yes, either abort startup or somehow "connect to it"