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.