Added support for toggle breakpoints

Also added rendering of the breakpoints
Added temporary toggleBreakpointsCurrentLine
Updated some comments
Various fixes

* Removed some magic constants
* Made it behave a bit more linke Sublime regarding the line area and markings
* Made it possible to toggle breakpoints on/off per line (not shown yet)
Made it possible to check if plugin is active
Changed STEP_INTO to TOGGLE_BREAKPOINT
ToggleBreakpoint change
Function for isActiveWindow
Support for toggling breakpoints

Also changed the way how event writers/readering is handled. Now is two buffers that gets swapped in order to prevent double updates.