Remove all trailing spaces

No real changes, just clean up sources by removing trailing spaces from
all the non-generated files.

This should hopefully avoid future commits mixing significant changes
with insignificant whitespace ones.

docs/doxygen/mainpages/const_stdevtid.h

@@ -29,7 +29,7 @@ identifiers are all in the range between @c wxID_LOWEST and
 @c wxID_HIGHEST and, accordingly, the user code should avoid defining its
 own constants in this range (e.g. by using wxNewId()).
 
-Refer to @ref page_stockitems "the list of stock items" for the subset of standard IDs 
+Refer to @ref page_stockitems "the list of stock items" for the subset of standard IDs
 which are stock IDs as well.
 
 */