provide for italic headings

[karlkleinpaste]

Some publishers prefer that their headings be produced as bold italic, not just bold, such as NET Bible, see image. We can't follow every publisher's desires, and we must have a generic default, as what naturally comes from Sword's filters (bold), but we can tweak this with CSS as we put it on the screen if the user wishes, with the knowledge that the filters produce "h3"-tagged headings.

This should be a popup-selectable choice. It should not appear in the popup if Headings are not already generally enabled. It should be available as well in all parallel displays (detached full chapter, tab full chapter, single verse parallel tab from standard).

Honestly, this small feature is in part an excuse to get my head back into the code after far too long.

[karlkleinpaste]

Along the way, I found that some bad code reformatting had been done in gtk/menu-popup.c and main/settings.c; this has been fixed so it again has neatly lined-up conditionals.
Also, a long series of "if" tests in main/parallel-view.cc were tweaked from "if" to "else if" because it's just testing the options string, one after the other.
And lastly, I discovered that Morpheme Segmentation had been left out of one of the .gtkbuilder files long ago, which I've added for completeness.