Memory Map improvements

[negasora]

• [ ] Warn when adding a segment/section not backed by file (will be obviated by #920)

(suggestions from https://github.com/Vector35/binaryninja-api/issues/1698#issuecomment-1222580558)

• [ ] Add a "Create Section" right-click menu option to the Segments pane that opens the Add Section dialog populated from the current segment
• [ ] Pre-populate the Add Section dialog with the selected segment's bounds
• [ ] Maybe a way to create a section for the parts of a segment that don't have a section defined on them