Editor: support toggling breakpoints on/off and removing all by menu command

[ivan-mogilko]

Sometimes it may be desired to run the game without stopping at breakpoints, but keeping them for the future use.

I propose to support a "disabled" state for breakpoints, and let toggle it both individually and in batch.

1. Breakpoints may have enabled or disabled state.
2. Disabled breakpoints have a different look. For example, in MSVS they are drawn as empty circles.
3. Add menu commands:

• Enable all breakpoints;
• Disable all breakpoints;
• Delete (or Remove) all breakpoints;

4. Figure out a convenient method for toggling an individual breakpoint, using a mouse, key combination or a context menu, etc.

If #2384 is implemented, I suppose that breakpoint's state also has to be serialized.