Feature/trace widgetry tweaks

A bunch of front-end-only changes for the traces page. Key changes include

 - The treemap 'drawer' now appears like a column in the packages list when it slides out. It no longer blocks the 'Coverage Barchart' column, which has been renamed to '% Coverage', and receives slightly more width when compacted.
 - All Activity and Overlaps no longer look like recordings. Instead, they are more like legend entries. The only interaction available with them is to change their colors.
 - The "Latest X" recording is now separate from the "All Activity" recording. It also doesn't get the little 'movie camera' icon when the recording is running. Removed the "Show" prefix from its label (now it fits on one line).
 - The "Start a Recording" button now lives directly under the "Latest X" widget, and has a slightly different look.
 - The default selection state (no recordings selected) for the page has changed. Now when there are no selections, the page behaves as if 'All Activity' is selected. After making selections, the treemap will still color nodes with the 'all activity' color (slightly faded) when those nodes are not covered by any of the actual selections.