Morphic: Prevent default action for ctrl-/cmd-key events

… and allow using these for Snap! without having Chrome trigger its own
actions for them
Snap.html: Focus the world canvas on startup

so Snap reacts to keyboard events right away (without having to click
on the IDE first)
Threads: new Variable data structure

for refactoring upvar references, not yet used anywhere
Search Blocks feature, keyboard shortcuts

Thanks, Kyle, for designing and architecting the search blocks feature!!
prepare release

correct last-changed-date
merged newest pull request
fixed conlficts that I missed the first time
Merge branch 'jmoenig-master' into graphics-ava