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
Merge pull request #964 from simonbaird/master

Add span wrapper around button caption text