Begin conversion to custom elements

[dethe]

Start with the high-level UI components (like the new Splitter) and make them using custom elements using the document-register-element polyfill.

See Why web components are ready for production and A W3C Custom Elements Alternative for more context on why this is a good time (and a good library) for this implementation.

[dethe]

So yeah, I've dug myself into the big web components rewrite and now I need to get it all working in time for January UCOSP students to work on it. Some things that remain:

• [x] Drag and drop
• [x] Scratch space
• [x] Accordion Widget
• [x] Replace block menu with trash icon while dragging (https://commons.wikimedia.org/wiki/Category:Trash_container_icons#mediaviewer/File:Human-user-trash.svg)
• [x] Support multiple types on values
• [x] Icons on expressions
• [x] Runtime functions (defining as we go)
• [x] Scope stack in runtime (invisible scope)
• [x] Run script
• [x] Define control blocks
• [x] Define sound blocks
• [x] Define array blocks
• [x] Define boolean blocks
• [x] Define color blocks
• [x] Define math blocks
• [x] Define random blocks
• [x] Define vector blocks
• [x] Block locals
• [x] Save and restore script
• [ ] Scoped dragging (visible scope) #884
• [x] Dialogs that work across desktop and mobile: https://github.com/kylepaulsen/NanoModal #858
• [x] Wire up the menu buttons #858
• [ ] Define sprite blocks #885
• [x] Define image blocks
• [ ] Define canvas blocks (???)
• [x] Define object blocks #886
• [x] Define string/text blocks #864, #883
• [ ] Define path blocks #887
• [x] Define point blocks
• [x] Define rect blocks
• [ ] Define (and rename) sensing blocks (mouse, keyboard, timer, stage) #888
• [ ] Define motion blocks #889
• [ ] Define shape blocks #891
• [ ] Define geolocation blocks #890
• [ ] Highlight current selection #892
• [x] Splitter dragging #855
• [x] Splitter state retention #855
• [x] Splitter double-click #855

Next round:

• [ ] Click/tap from menu to add block to selected block #892
• [ ] Click/tap on block or contains to change selection #892
• [ ] Highlight potential drop targets #893
• [ ] Create variable when expression is dragged to a contains #894
• [ ] Icons on values #895
• [ ] Type colors on blocks and values #896
• [ ] Highlight current drop target #893
• [ ] Visualize scope (grey out illegal drag targets) #893
• [ ] Move all user-visible text into HTML text (not attributes, not CSS, not JS) for localization #897
• [ ] Make sure we can accomplish anything (short of 3D) that Processing can #898
• [ ] Rich expression blocks to create colours, dates, arrays, objects: collapse like contexts #899
• [ ] Search #856
• [ ] Cut-and-paste (C-C, C-V, C-X) where C- is either Ctrl (windows) or Command (mac) #900
• [ ] Undo/redo (C-Z, C-Shift-Z) #663

[dethe]

All remaining checklist items are now promoted to issues, closing this.