Removing xforms cruft

[Jean-Marc Lasgouttes]

One of the (bad) reasons for including forms.h is the need to access
fl_display() and fl_screen(). What is the best place to get those
things? Would adding LyXGUI::x11Display() and LyXGUI::x11Screen() be a
good idea?

On a related subject, do we want to dump custom export (aka SendTo),
since it can be obtained by creating a new export target?

JMarc