Reduce threading (#685740)

Win_Gparted and Dialog_Progress were creating threads to perform most
functions in the background.  Most of the time, the only reason the
threads blocked was to execute an external command.  The external command
execution has been changed to spawn the command asynchronously and wait
for completion with a nested main loop.  While waiting for completion,
the pipe output is captured via events.  In the future, this will allow
for it to be parsed in real time to obtain progress information.

Those tasks in GParted_Core that still block now spawn a background thread
and wait for it to complete with a nested main loop to avoid hanging the
gui.

Part of Bug #685740 - Refactor to use asynchronous command execution