#897 option to display rectangle

[rayzhuca]

Description

Created a new display option for base pairs: rectangle.
Moved changed the display into a dialog box.

Screenshots (if appropriate):

The dialogue box.

The display when display with rectangle and even if mismatched as false.

[dave-doty]

I'd actually prefer a slightly different way of specifying view options.

Instead of popping up a dialog, can we make it all configurable from the submenu View-->Base pairs, similar to View-->Modifications? There, if "Show modifications" is unchecked, that's all you see:

but then if you check it, you see the additional options:

That's what I had in mind for these options, if unchecked, it looks like it did before, and if checked, then you see something like this directly in the submenu:

I don't know if the React Bootstrap library we are using for submenus allows radio buttons, but if not we can just have "lines" and "rectangle" be two checkboxes, and set up the React render code so that exactly one of them is checked (according to the value AppState.AppUIState.base_pair_display_type)

[rayzhuca]

I couldn't find the radio buttons so I just made it into checkboxes like you suggested.