Minor cleanups to Haiku menu code

Minor cleanups to Haiku menu code

* src/haiku_draw_support.cc (BView_SetHighColorForVisibleBell):
Delete function.
* src/haiku_font_support.cc (BFont_dat): Rename to
`BFont_metrics'.
* src/haiku_support.cc (DrawContent): Use correct UI color for
control text.
* src/haiku_support.h (HAIKU_MODIFIER_ALT): Turn into enum.
(enum haiku_modifier_specification): New enum.
* src/haikufont.c (haikufont_open, haikufont_close): Fix coding
style.