Add span wrapper around button caption text

Reasons:
- can show or hide the button text with CSS (assuming
  tv-config-toolbar-text is yes).
- can have different looking buttons in the page controls versus the
  view toolbar, etc
- more flexibility styling the button appearance, for example you
  can change the text size compared to the icon size
- button appearance is more themeable
WIP: mptw5 hacking

Progress:
- Removed the popup menu when you click a tag
- Shows a subtree of tagged tiddlers
- Added button captions by default and smaller buttons
- New here buttons enabled by default

Todo:
- Add tag popup as extra menu