Auto feature for minibuffer toolbar

Auto feature for minibuffer toolbar

Now the minibuffer toolbar is "auto" by default. It is opened by
command-execute (M-x) and closed when the command is executed without error.

* make lyx::dispatch return a DispatchResult struct

* there is a new MINIBUFFER type of toolbar, that can be used for this use.

* remove special handling of M-x in minnibuffer; Escape can be used instead. Fix focus in this case.

* when minibuffer toolbar is "auto", make the toolbar close itself after
  - a command has been executed without error
  - an empty command has been executed
  - the Escape key has been used

[this is actually commit fdcff02a, which was later reverted at dd61d8cf]