Make main_ui be heap allocated

This is preparation for being able to create more than one UI object.

The change to gdb_main to stop using catch_errors is necessary because
catch_errors references current_uiout, which expands to
current_ui->m_current_ui, which would crash because current_ui is not
initialized yet at that point.  It didn't trigger earlier in the
series because before this patch, main_ui/current_ui always start out
non-NULL.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* event-top.c (main_ui_): Delete.
	(main_ui, current_ui, ui_list): No longer initialize here.
	* main.c (captured_main): UI initialization code factored out to
	new new_ui function.
	(gdb_main): Wrap captured_main with TRY/CATCH instead of
	catch_errors.
	* top.c (highest_ui_num): New global.
	(new_ui): New function.
	* top.h (struct ui) <num>: New field.
	(new_ui): New declaration.

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