Improve "Untrusted Application Launcher" Dialog

Followup of !320 (merged)

Untrusted Application Launcher Dialog

• The message is not always appropriate, because the file may be executable but not set as trusted. I guess in practice this isn't a big deal, but it's a little weird when the file is executable in the first place, as it is with Tor Browser.
• The "Mark Executable" button could be called "Mark Executable and Launch", because next to "Launch Anyway" you wonder if it will actually launch the app or not (this is something I've thought about before, about desktop launchers).