Add a touch_ui_dialog_modal() function which in some way displays an
error or other message to the user, and possibly offers the
possibility to choose some "OK", "Cancel", "Retry", "Ignore" type of
response. Note that the "modality" is from the point of view of the LO
thread.

Change-Id: I1b2ad15854f8d76e2c30f93fb5356fad7e5d02ca