1. Expanding divs to improve text selection. (Yury)
2. Adding enhanceTextSelection as an option.
3. Moving feature functionality from text_layer_builder.js to text_layer.js.
4. Added expandTextDivs method to only load expanded divs on first click, and only show on subsequent clicks
These have been found using `gulp lint` in combination with the `unused:
true` parameter for JSHint. Unfortunately there are too many false
positives to enable this feature, but now that most globals have been
removed because of the conversion to UMD the results are much more
useful than before.
Some browsers render certain special characters with width 0, others with strictly positive width. (For example, the Greek Delta, Δ, has width 0 in Google Chrome, and a positive width in Firefox.) The `if` clause in operation so far results in different text layer DOM trees for different browsers.
This commit fixes that by adding the elements independently of their width.
