Draw spaces on current line option

[jeremypw]

Fixes #570

Adds a third draw-space mode as requested. In this mode spaces are always drawn on the current line. They are also drawn on any selections. To allow the user to choose between three states, a switch is replaces by a combobox.

[zeebok]

I am noticing that if I have whitespace at the end of a line, it will still draw the spaces when on Current and on a different line:

[jeremypw]

@zeebok Looks like the same thing happens in master. If it can be fixed here without significantly increasing the diff I will.

[jeremypw]

@zeebok: Looks like this is deliberate behaviour (when draw spaces is ON at all) - see #433