xpra session file launch fixes

[totaam]

Every platform needs fixes I think:

• on win32 I get registry warnings when running ccleaner (and I think it may remove our file association if you ask it to fix it!)
• on Linux, we are missing the mimetype association for the launcher and the launcher desktop file triggers this lintian warning: W desktop-mime-but-no-exec-code
• osx is already tracked in handle .xpra file associations on osx #407 (not implemented yet)

[totaam]

For Linux, following the very helpful guide: Add a custom MIME type for all users, we now add the mimetype file in r9571 and everything seems to work.
This allows us to double-click on a ".xpra" file to fire up the launcher (and optionally auto connect if autoconnect=true is set in that file).

[totaam]

I cannot reproduce the problem I had seen on win32, will follow up for osx in #407 and #641.

(this would be safe to backport)

[totaam]

2015-06-02 12:08:19: antoine uploaded file test.xpra (0.1 KiB)

most basic launch file I have used for testing