Enable spellchecking on Windows 8.

Update wxWidgets to a version that doesn't stand in the way by using
EVT_CONTEXT_MENU.  Enable IMF_SPELLCHECKING support in rich edit control
v8.

Notice that this only works on Windows 8+, earlier versions didn't have
the spellchecking API or rich edit control support for it.  On earlier
versions of Windows, everything would have to be done manually,
including obtaining dictionaries.