Add feature: Press space to rotate overlapping hints

Also noted the feature in the help dialog.

And while cleaning up the added code in marker.coffee, I couldn't stop
myself from cleaning up the rest of the code there a bit, too.
Merge branch 'develop' into hints-overlap-fix

Conflicts:
	extension/locale/ru/vimfx.properties
Move `z-index` out of `.VimFxReset`

The hint markers need to set `z-index` dynamically (to rotate overlapping
markers). style.css is inserted as an agent sheet and uses `!important`
everywhere (this is needed to override userstyles, see commit 6145382),
and can therefore not be overrided by inline styles.

Therefore, I've moved `z-index` out of `.VimFxReset`, to each place that
actually _needs_ it: A few positioned elements.
Fix: Overlapping hints rotation was mostly broken
Make marker border more prominent
Merge branch 'hints-overlap-fix' of git://github.com/lydell/VimFx into lydell-hints-overlap-fix
Enabled Shift-space to rotate hint markers backwards
Small code tweak
Code Review
Merge pull request #1 from akhodakivskiy/lydell-hints-overlap-fix

Enable forward/back marker rotation
Merge pull request #178 from lydell/hints-overlap-fix

Add feature: Press space to rotate overlapping hints