remove wxSYS_DEFAULT_PALETTE: it's supported only by wxMSW and it never worked there...

remove wxSYS_DEFAULT_PALETTE: it's supported only by wxMSW and it never worked there: GetStockObject(DEFAULT_PALETTE) returns a palette and not a font; so the resulting wxFont is wrong.
Expand and correct documentation for various system constants.
Provide wxSYS_COLOUR_FRAMEBK as a synonym for wxSYS_COLOUR_BTNFACE as it is the colour which must be used to make wxFrame background coherent with the background of the controls they (eventually) contain.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59313 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775