debugger: Refactor breakpoint handling.

The breakpoints are now (fairly) tightly guarded by the BreakpointHandler.
Engines and Views are only supposed to refer to them by id. They also have
individual states now. The breakpoint data is split into a "user requested"
"fixed" part in BreakpointData and the engines' acknowledged data in a new
struct BreakpointResponse.

TODO: Move m_state and m_engine members to BreakpointResponse. Fix regressions
in the marker handling.