DIALOG_EXIT was not being laid out correctly after setting the dialog
message. Rather than fix our custom DIALOG_EXIT, wxMessageDialog and
wxRichMessageDialog were substituted as direct replacements for all
of the variants of DIALOG_EXIT. This make message dialogs appear more
consistent because wxMessageDialog uses the default platform message
dialog and wxRichMessageDialog uses the default platform rich message
dialog on windows.
Remove DIALOG_EXIT_BASE as it is no longer required.
Fixes lp:1832899
https://bugs.launchpad.net/kicad/+bug/1832899
(cherry picked from commit 3f665318f4ab5c02b7d3c29b5233a273fdac2e4e)
8002692f3e