altosui: Clean up packet link connecting dialog

Make sure the dialog is destroyed after use (otherwise, it hangs
around on the screen sometimes).

Switch timeout before showing dialog to 500ms -- that brings the
dialog up less often when unnecessary.

Use 'timeout_started' boolean to indicate whether the I/O thread has
queued the dialog for display and whether it needs to queue a call to
close it down.

Signed-off-by: Keith Packard <keithp@keithp.com>