Minor fixes/improvements related to GUI

[Jan Wassenberg]

• We now get warnings whenever there is a C++ file that is empty or doesn't contain any meaningful info (e.g. stuff that would end up in the compiled static library). Accordingly: decide if CGUIScrollBarStyle.cpp and CGUIList.cpp can be removed.

• Some parts of the GUI still use the old and obsolete PS_ERROR mechanism. Removing that would be nice, to avoid confusion. Also, localization would be helped a bit by not having error text hard-coded in English.

If a return-value-based approach is still desired (instead of tossing exceptions), we could simply assign numbers to the error definitions, rename them a bit, and be done. Otherwise, we can move that over to Philip's error mechanism, i.e. throwing exceptions. I can help out and/or do all the work - just let me know which is preferable. (old TODO entry: move GUIbase.h errors (DECLARE_ERROR) into e.g. Philip's error mechanism, remove DECLARE_ERROR, remove ps/errors.h.)