Work around some gtk oddity with gtk_combo_box_entries and empty text

Work around some gtk oddity with gtk_combo_box_entries and empty text

Setting the gtk_combo_box_entry test to th eempty string doesn't "take":
the old text remains.  Which does all kinds of funky things when you
switch between dives, and the location (or buddy or divemaster) entry
contains some random stale entry from another dive.

This works around it by using a string with a single space in it
instead, and then removing the space when reading.  Not pretty, and
certainly not correct, but it pinpoints the odd behavior.  I'm sure
somebody will figure out what the magic gtk incantation is for this.

Also remove the never-used flags for whether the entries have changed.
They were designed to be set by change callbacks, but we never bothered
with it, and just always read the value of the entries instead.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>