Fix SlotAsset context construction for unpositioned assets

Fixes #2937
Make clicking outside timeline selection clear selection

However, this doesn't work within the video play bar.
Fixes #2938
Upgrade play-plugins-mailer to 2.3.1
Merge branch 'master' into spreadsheet
Start adding assets to spreadsheet

Only minimal display so far, no editing.  The main question is how to
populate slot.assets.