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.)

[sluzky]

deleted the remaining CGUIList.cpp

[sluzky]

(In [7147]) refactoring all PS_RESULT in GUI to PSRETURN/PSERROR closes #128

[sluzky]

gonna buff up the log for a while until all GUI settings are fixed

[(none)]

Milestone ASAP deleted

[elexis]

In 23067:

Don't leak all GUI objects on name ambiguity following rP9 and remove ERROR_GROUP GUI.

Delete CGUI::UpdateObjects and don't recreate the entire object hierarchy by recursing through the existing one each time when inserting one GUI Object.
Change CGUI::AddObject to only insert the one relevant pointer into the map hierarchy, return success value and delete the child upon failed insertion in the caller instead of leaking it.
Defer GUIM_LOAD and UpdateCachedSize recursion until all XML files linked in the page XML file were loaded.
Delete related ERROR_TYPE NameAmbiguity, ObjectNeedsName, and unrelated JSOpenFailed and in consistency with the rest, do the LOGERROR in the scope where the error occurs.
Delete ERROR_TYPE InvalidSetting from rP7145 unused following rP22604, rP22796, ERROR_TYPE OperationNeedsGUIObject from rP7145 unused following rP22641, see also rP22741, refs #128.