Re-highlight windows when needed during TUI startup

Re-highlight windows when needed during TUI startup

I noticed that "tui enable" did not correctly show the source window
as having the focus.  Debugging showed that the problem was that
tui_update_variables was called after the windows were drawn, and its
result was being ignored.  This changed the code to re-highlight the
windows if the value changed.

gdb/ChangeLog
2019-12-01  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_enable): Call tui_update_variables earlier.

Change-Id: I1a4563fb431833dd3211a224c9e2df3b936fe9ce