modeman: Make it possible to override a locked mode.
prompter: Override locked mode for questions.

This fixes a crash ([1], "ValueError: Mode KeyMode.yesno not on mode stack!")
when a website asks a second javascript question (prompt/alert/confirm) while
another one is still running.

[1] http://paste.the-compiler.org/view/8c39f5ea
statusbar: Ignore subsequent _show_prompt_widget.

If it's already shown, we don't want to mess with stuff again.
prompter: Show prompt when restoring context.

This fixes being locked in prompt mode with no prompt when a website asks a
javascript question while another one is still visible.
Move show_prompt/hide_prompt signals from prompt to prompter.