Allow specifying font size in the Haiku font selection dialog

Allow specifying font size in the Haiku font selection dialog

* src/haiku_support.cc (struct font_selection_dialog_message):
New fields `size' and `size_specified'.
(class EmacsFontSelectionDialog): New `size_entry' control.
(MessageReceived): Set size from that control if available.
(EmacsFontSelectionDialog): Initialize that control.
(FrameResized): Handle layout for that control.
(be_select_font): New field `size'.
* src/haiku_support.h: Update prototypes.
* src/haikufont.c (Fx_select_font): Populate font spec with
size.