Toggle data pane

[rolyp]

Implementing #980 makes the UI a bit flickery when moving between e.g. bars in a bar chart (assuming nothing else selected), as the record set disappears when the mouse moves off a visual element. It would be better to show the empty record set, and then separately have a feature that allows the user to toggle the display of underlying data on/off.

• [x] “Data pane” icon, pale by default – use Unicode characters rather than Font Awesome icon
• [x] Enabled when icon pressed
• [x] Attach mouse listeners specific to particular row of grid
• [x] Button to hide/reveal left column
• [x] Data pane disabled by default
• [x] Visual cue that left column is “different”
• [x] :hover style for data pane toggles

See also:

• 988

• 858

[rolyp]

Reopened, re-scoped after implementing #988