Fixed editor toolbars not being initialised correctly.
Fixed menu icon spacing.
Removed plugin card menu hack.
Implemented scrollable tabs in plugin editor.
Implemented animated tab page changes.

The tab pages in the editor are zero-height though, I couldn't figure
out how to solve that, so I'll just go back to it later. The Polymer
elements are in, and that's what matters at this stage.
Added header overlay back in.

For disabling the toolbar use during editing.
Added toasts for feedback, removed dead function.
The last selected menu item shouldn't stand out.