Implement #9777.

- Introduce class BreakpointProxy which acts as a container for
either a breakpoint or a watchpoint. BreakpointsTableModel now stores
a single list of these rather than separate Breakpoint/Watchpoint lists.
- Switch BreakpointListView to allow multiple selection mode, and
consequently change selection/listener interfaces to use a list of
BreakpointProxy objects. Adjust implementors accordingly.
- Rework breakpoint list columns to better mesh with a unified display
of breakpoint and watchpoint information.
- Add an input filter to handle removing breakpoints when the delete
key is pressed.