Remove unnecessary CSS classes when specifying button icons
Remove unnecessary CSS classes when specifying button icons

Currently we're using *both* ids and classes when specifying the button icons, which seems completely unnecessary and only serves to bloat the CSS code.
In general you always need to be careful about CSS specificity, but in these cases that should not actually be a problem.

Also, while not a button, this patch makes a similar simplification for the `pageNumber`-input.

Shoobx/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js