Use GLib log/print handlers to log messages/warnings into a string for later usage.
Add a debug messages window, accessible through the help menu to easily view debug messages/warnings.
This might replace the extra console window on Windows.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@2876 ea778897-0a13-0410-b9d1-a72fbfd435f5