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)
Merge remote-tracking branch 'origin/58385358-panels-like-mockup' into testing