add a way to suspend the subiquity UI and run some other command

This will be used for the drop to shell functionality and also to allow
viewing an error report before we submit it to daisy.

It is almost but not quite surprisingly easy (I added a long-ish comment
about the difficulties).

subiquitycore/core.py

@@ -19,6 +19,7 @@ import json
 import logging
 import os
 import struct
+import subprocess
 import sys
 import tty
 
@@ -297,6 +298,28 @@ class Application:
             os.write(pipe, b'x')
         fut.add_done_callback(in_random_thread)
 
+    def run_command_in_foreground(self, cmd, **kw):
+        screen = self.loop.screen
+
+        def run():
+            subprocess.run(cmd, **kw)
+
+        def restore(fut):
+            screen.start()
+            # Calling screen.start() sends the INPUT_DESCRIPTORS_CHANGED
+            # signal. This calls _reset_input_descriptors() which calls
+            # unhook_event_loop / hook_event_loop on the screen. But this all
+            # happens before _started is set on the screen, so hook_event_loop
+            # does not actually do anything -- and we end up not listening to
+            # stdin, obviously a defective situation for a console
+            # application. So send it again now the screen is started...
+            urwid.emit_signal(
+                screen, urwid.display_common.INPUT_DESCRIPTORS_CHANGED)
+            tty.setraw(0)
+
+        screen.stop()
+        self.run_in_bg(run, restore)
+
     def _connect_base_signals(self):
         """ Connect signals used in the core controller
         """