Replace wxT() with wxS() in image options definitions.

Replace wxT() with wxS() in image options definitions.

The optimal way of creating a literal strings consisting of only ASCII
characters is to use wxS() as this avoids the need for run-time conversion in
all builds while wxT() requires a conversion from wchar_t to UTF-8 in UTF-8
builds. So replace wxT() with wxS() in wxImage options definitions.

Also remove wxT() from the options documentation, it's just useless there.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69760 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775