Begin conversion to custom elements

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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.

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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:

• Drag and drop
• Scratch space
• Accordion Widget
• Replace block menu with trash icon while dragging (https://commons.wikimedia.org/wiki/Category:Trash_container_icons#mediaviewer/File:Human-user-trash.svg)
• Support multiple types on values
• Icons on expressions
• Runtime functions (defining as we go)
• Scope stack in runtime (invisible scope)
• Run script
• Define control blocks
• Define sound blocks
• Define array blocks
• Define boolean blocks
• Define color blocks
• Define math blocks
• Define random blocks
• Define vector blocks
• Block locals
• Save and restore script
• Scoped dragging (visible scope) Implement scoped dragging for locals #884
• Dialogs that work across desktop and mobile: https://github.com/kylepaulsen/NanoModal Wire up buttons to open dialogs (development) #858
• Wire up the menu buttons Wire up buttons to open dialogs (development) #858
• Define sprite blocks Define sprite blocks #885
• Define image blocks
• Define canvas blocks (???)
• Define object blocks Define object blocks (development) #886
• Define string/text blocks Implement string blocks in development branch #864, Implement the Text blocks #883
• Define path blocks Define path blocks #887
• Define point blocks
• Define rect blocks
• Define (and rename) sensing blocks (mouse, keyboard, timer, stage) Define and rename sensing blocks #888
• Define motion blocks Define motion blocks #889
• Define shape blocks Define shape blocks #891
• Define geolocation blocks Define geolocation blocks (development) #890
• Highlight current selection Highlight current selection #892
• Splitter dragging Splitters don't drag or resize #855
• Splitter state retention Splitters don't drag or resize #855
• Splitter double-click Splitters don't drag or resize #855

Next round:

• Click/tap from menu to add block to selected block Highlight current selection #892
• Click/tap on block or contains to change selection Highlight current selection #892
• Highlight potential drop targets Highlight drag targets while dragging #893
• Create variable when expression is dragged to a contains Create variable block when expression is dragged to wb-contains #894
• Icons on values Add type icons to empty values #895
• Type colors on blocks and values Blocks should be the colour of their menu #896
• Highlight current drop target Highlight drag targets while dragging #893
• Visualize scope (grey out illegal drag targets) Highlight drag targets while dragging #893
• Move all user-visible text into HTML text (not attributes, not CSS, not JS) for localization Move all user-visible text to HTML text #897
• Make sure we can accomplish anything (short of 3D) that Processing can Add blocks to support Processing API #898
• Rich expression blocks to create colours, dates, arrays, objects: collapse like contexts Add blocks to support Dates and date processing #899
• Search re-implement search #856
• Cut-and-paste (C-C, C-V, C-X) where C- is either Ctrl (windows) or Command (mac) Implement cut/copy/paste #900
• Undo/redo (C-Z, C-Shift-Z) Re-implement Undo & Redo #663

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