MainWindow: make tabs pinnable

[cassidyjames]

Intended to fix #29. I actually came to file a similar issue to help me clean up my tab bar when I have a few long-running process, e.g. web servers, and found it was already reported. So far, this draft just makes tabs pinnable but does not prevent changes to the tabs themselves.

Icon-wise, I was looking for a pinned icon but found changes-prevent and view-pin. Neither seem like the perfect fit, but either might work fine. Open to feedback. I am seeing an issue where the icon is not centered in the pinned tab; not sure if I'm doing something wrong, or if this is a bug in Granite or the stylesheet.

• [x] Make tabs pinnable
• [x] Update icons
• [ ] Prevent closing pinned tabs?
• [ ] Restore state of pinned tabs