Overhaul tags box

- Improvements to #20, with the tags box switching to a multiline
  textbox in the style of #164 on a multiline paste or Shift-Return. In
  the multiline box, Return is a newline and Shift-Return saves
- Allow tabbing through tags via keyboard (and keep the last empty
  textbox open on tab, so you can hold down the tab key to get all the
  way to the end)
- Fix various post-update focusing issues (though the wrong textbox is
  still selected for some multiline updates via Tab/Shift-Tab)
- Make (single-line) tag entering much faster by not reloading the whole
  tags list and just placing the new tag in the correct sorted position.
  This could be made even faster with tag selector optimizations.
- Allow the Add button to focus when switching to the Tags pane (and the
  same for the Related pane, for good measure)