Make the sidebars/panels more like in mock-up

- Bar only appears when the panel is collapsed
- When the bar does appear, the whole thing is clickable (as is the
panel heading)
- "Add" dropdown moved into panel to avoid conflict with collapse
function
- Many style changes (including much more variable reuse instead of
static px), some just to get new basics working, and some to make things
consistent/better
- Training wheel arrow doesn't spill over to TOC due to overflow issues
(to address later)
fix picker background issue that will appear with "add" button's appearance
allow click anywhere on "add" dropdown line

It's obnoxious to have to have your mouse exactly over the exact text of
the option in the "+ Add" dropdown in order to make it work (as opposed
to clicking anywhere on that option's line). This should fix that.
Please excuse my committing this to an unrelated branch. :see_no_evil:
allow for different widths for picker vs. TOC

Split @aside-width into @picker-width and @toc-width so that the panels
can be made different sizes (I made the picker a little smaller). The
.x-indent-compensate stuff is _mostly_ the same (minus some width
adjustments), but I had to move it out of its former context in order to
pass the panel widths through.
Merge remote-tracking branch 'origin/master' into 58385358-panels-like-mockup
remove commented out code

not sure why that got left in, sorry!
Merge pull request #73 from oerpub/58385358-panels-like-mockup

Make the sidebars/panels more like in mock-up