Fine-tune multiple dialog instance handling

prevent multiple block editors on the same block definition, allow
multiple dialogs on different objects, handle dialog instances in
DialogBoxMorph.prototype
Hide Primitives feature

Primitive blocks in the palette can now be hidden in the project via
their context menu. Each palette's context menu further lets you hide
or show all its primitives depending on whether any primitives are left
to hide or show.
Hidden primitives are stored in the project data. This lets instructors
create "simplified" examples and problem sets.
Restore multi-dialog restrictions for multi-world setups
Translation update for "hide primitives" feature
Ensure unique World stamps
Paint editor tools symbols

additional simple icons for buttons: brush, rectangle, circle, line,
crosshairs
Paint bucket symbol for paint editor flood-fill tool
Fixed #70
Solid rectangle and circle symbols
Costume shrink-wrapping
Fixed #83
Eraser symbol
Fixed #24
Fixed #24 for horizontal scrolling
Merge pull request #84 from queryselector/issue-24

Fixed #24 for horizontal scrolling
Reset Password Feature

(front end only)
Reset Password via emailed link

(front end only)
Paint editor

contributed by Kartik Chandra (Hardmath123 on Scratch)!!
Paint editor fixes
Paint editor integration