use native Gtk2 crash dialogs on Linux

closes #3872
closes #3849
condense into one graphical crash dialog

make it more robust against missing dependencies
copy launch scripts during Linux make install
document the launch scripts
fix missing hard-coded native deps after make install
always spawn the native error dialog
GitHub URLs have changed

https://developer.github.com/changes/2014-04-25-user-content-security/
put all remaining WinForms targets together

so it is easier to get rid of them
don't ship OpenRA.CrashDialog.exe on Mac/Linux
clean up old Changelog files
Merge pull request #5252 from Mailaender/zenity

Added native Gtk+ crash dialogs on Linux