Added wxGetMouseState which returns the current state of the mouse.

Added wxGetMouseState which returns the current state of the mouse.
Returns an instance of a wxMouseState object that contains the current
position of the mouse pointer in screen coordinants, as well as
boolean values indicating the up/down status of the mouse buttons and
the modifier keys.  Implemented for wxMSW, wxGTK and wxMac.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@36691 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775