gui: Reorder functions.

gui: Reorder functions.

As a preparation for the removal of do_select() we need various
functions of gui.c to be in a different order. Specifically,
input_post_select() will soon call handle_command(), so the latter
function will need to be declared before the former.

Patches which move functions around are generally hard to
read, so this commit performs pure code movement only. However,
as exec_and_display_cmd() still calls do_select() we introduce a
temporary forward declaration for do_select(). It will be removed
in the next patch.