ITextMark is not abstract anymore and has an icon and a priority.
This means separate breakpoint and location marker classes that
are only "plain" marks with icons and priorities are not needed.
BaseTextMark directly inherits from ITextMark, instead of owning
an ITextMark derived InternalMark.
Also, there is now ITextMark::paint() to make it a bit more flexible
then icon()[->paint()]
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.
