Human interaction

[pufferfish101007]

Adds some new blocks, primarily to do with interacting with the user.
New blocks added:

• sensing_askandwait
• sensing_answer
• event_whenthisspriteclicked
• event_whenstageclicked
• looks_switchbackdropto
• looks_backdrops
• looks_backdropnumbername
• looks_nextbackdrop
• data_showvariable
• data_hidevariable
• control_stop all
• control_for_each (resolves control_foreach should be implemented #13)
• sensing_mousex, sensing_mousey, sensing_mousedown
• looks_costumenumbername with name field
• operator_round
• data_itemnumoflist

Also:

• fixes Project state goes a bit strange when navigating away without page reload #39 (75943a8)
• fixes nested loops with no block in between cause stack overflow in Display #52 (9d3bce7)
• fixes hq test project fails in SSA #53 (64770b6)
• fixes incrementing variable in 'repeat until' loop inside warped procedure gives wrong result #54 (5e9dc8d, 4780e38)
• maybe resolves compiling deeply nested non-warped if/elses is catastrophically slow #55? (8d9a61a)
• compiles project in a webworker to avoid hanging the main thread
• allows calling nonexistant procedures as noop (e.g. comment blocks, SA debugger blocks)

Still TODO:

• backdrops don't display at the correct position if the costume centre is not at the origin (this probably happens with sprite costumes as well?)
• event_whenbackdropswitchesto?