Support moving editor windows to a new monitor.

Hello all,

I recently posted a feature request for allowing source editors to be
opened in a new window (which can have it's own splits and tabs) on
bugs.kde.org, and it was suggested I contact the mailing list
instead. - ( https://bugs.kde.org/show_bug.cgi?id=472334 ) - So I'll
repeat it below:

This is a feature request to make editor windows detachable such that
they can moved onto a different monitor head. This new editor window
can have it's own tabs as well. So it would be like having a second
main window, but without the toolbar, menu, etc,...

This could be implemented by dragging tabs outside of the main
window, and/or having a context menu on right-clicking the tab with a
'move to new window' option, along with a corresponding entry in the
Window menu.

I took a look at the code to see how much work this would be, and
unfortunately, due to the way things are set up, with the Sublime
MainWindow owning a lot of the state and event handling around tabs,
it would not be very trivial.

In order to implement this, we'd need to abstract all of the
tab-specific code into a type of TabContainer Widget or Window base
class and add a way of tracking the currently active/focused tab
container/window so we know which one to address when a new tab is
requested for instance. Moreover, all the tab related event/signal
handling functions would also have to be moved, as the sublime
MainWindow currently has stuff like TabDoucleClicked() etc,..

As I had mentioned in the original issue on the bug tracker, I don't
mind taking a stab at the implementation of this myself, but Igor was
concerned about being the only reviewer on this feature. As such,
would others be interested in reviewing or help developing this?

Thanks,

-jns