Prevent the zoom dropdown icon from being overridden when the element is `:active` (bug 1619595)
Prevent the zoom dropdown icon from being overridden when the element is `:active` (bug 1619595)

This changes the dropdown icon from being set using the `background` CSS property, to being set with `::after` which is *similar* to all the other toolbar button icons (which use `::before`).
Also tweaks the dropdown `background-color` on `:hover` slightly, since the other changes made it too light.

Snuffleupagus/pdf.js,xavier114fch/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,caremerge/pdf.js,mozilla/pdf.js,macroplant/pdf.js,macroplant/pdf.js,caremerge/pdf.js,macroplant/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js