Bump configure.in to require Mono 3.0.4+
Use mono-sgen for the Linux launch scripts

Fixes BXC12010 - mono option --gc=sgen is silently ignored
Don't run Mono in debug mode by default on Linux
Set OXYGEN_DISABLE_INNER_SHADOWS_HACK in launch script

Fixes blank text editor when using oxygen-gtk theme.
[Ide] Improve filtering of bad GTK+ themes

* Case-insensitive check
* Allow oxygen-gtk if OXYGEN_DISABLE_INNER_SHADOWS_HACK is set
[Ide] Improve check for bad GTK+ themes on startup

* Actually check the current theme, not just the preference
* The Xamarin theme is the default fallback, if installed
* Tell the user to install additional themes if there aren't any
* Allow running with bad themes if no fallback exists
Update gitignore