Close tab to the left or right

[ObserverHerb]

Vivaldi has a nice feature where you can close tabs to the left or right of the current tab. I wanted it in Otter so I added it, then figured might as well open a pull request for it.

I'd like to not show it on the left-most or right-most tabs, but I'm not sure how to get the tab count from within TabBarWidget.cpp.

[Emdek]

@herbmillerjr, for now we can get it pretty easily, but we need to think about plans from #190, namely tab stacking.
First we need to decide how it should behave when called on tab that belongs to stack, should it close tabs inside own group or from parent group as well?
Also we need to decide if these actions should have own identifiers or be a special variant of CloseOtherTabsAction, or some new entry but shared by both variants (any ideas how we could name it in this case?).
Last but not least, we are preparing to make that menu customizable, so the right place to handle it would be just MainWindow and working with SessionModel might be the best way to handle it (TabBarWidget is in the process of becoming the slave of SessionModel).