Another Windows file dialog change in Rambetter-temp-fixes branch.

Another	Windows	file dialog change in Rambetter-temp-fixes branch.
Handling file extensions better	during save operations.	 This is more
Windows-compliant.  For	example, when saving map:
- If ".xmap" filter is selected	and user types "foo.map", map will be
saved as "foo.map" (previous behavior was "foo.xmap").
- If an	unrecognized file extension is typed, GtkRadiant will now honor	that
but issue a warning popup window: "Unknown file extension for this save
operation.  Attempt to save anyways?". 	All known extensions for the type
are checked.  Previous behavior	was that the extension is always changed
based on selected filter.

I will continue	making lots of incremental fixes to Rambetter-temp-fixes.

THIS PATCH SHOULD BE MERGED INTO TRUNK.

git-svn-id: svn://svn.icculus.org/gtkradiant/GtkRadiant/branches/Rambetter-temp-fixes@353 8a3a26a2-13c4-0310-b231-cf6edde360e5