Auto-completion in HTML panel

Assorted fixes for issues 3931, 3700, 6854, 4458, 6814. (Still wanted: auto-completion for "class" attribute; only including the SVG tags/attributes supported by Firefox.)

Anyone wanna try this out and see if anything feels buggy? In particular, behavior of the tab key within the "style" attribute is weird and needs some discussion. (And I guess Enter should be the same as Tab.)

Code comments are of course also welcome.