debugger: Better document line tracking

Continuing from !507 (merged)

The commits included in here aim to improve the breakpoint document line tracking, the document discard and save behavior.

For a reference: !147 (merged) was the last MR that did some work on the BreakpointModel, and in this MR I'm basically continuing that work.

Bit of terminology discussed in this MR:

• Document line tracking: a breakpoint can follow the document line in an open document as it is edited. Implemented mostly using the MovingInterface API.
• A breakpoint mark: the document border mark type (bookmarks, etc.) that is shown for breakpoint instances. Implemented using the MarkInterface API.
• Breakpoint: an instance of the Breakpoint class.