Random improvements to require statements.
Removal of some obviously dead CSS.
.blocklyDropDownDiv no longer supresses text selection (see comment in PR).
This PR drops the compiled size by 4 KB, which is larger than I was expecting.
Also removes three private accesses from workspaceSvg into toolbox tree.
The strategy for workspace.updateToolbox is no longer to clear and repopulate the toolbox, but rather to throw away the old toolbox and rebuild a new one. This is simpler and more reliable.
This commit trims off another KB from the compiled code.
