two reasons:
1) to keep performance acceptable playmode only checks halting
conditions on a CPU instruction boundary. however a CLK changes many
times during an instruction meaning it will never match.
2) a CLK breakpoint will always match within 228 emulation ticks so
there is no point entering playmode at all because it will definitely
drop back to the debugger (within microseconds)
added a range change check to SCANLINE and CLK targets in
breakpoints.parseCommand(). we know what the possible values are for
these targets so we can be helpful and inform the user the some values
will never match
halting conditions are checked every video-cycle. this reverts a
temporary change made in a73dcae6dcc61e4ced03f23fe4df504cb966fb36
renamed files in debugger package for clarity
