keyboard: Delegate enter/shift/layout/hide buttons style to theming

keyboard: Delegate enter/shift/layout/hide buttons style to theming

Drop the UTF8 glyphs from those, and add style classes so those can be
specifically themed and given a background image. The style classes are:
.keyboard-key.enter-key{}
.keyboard-key.shift-key-lowercase{} /* applies while lowercase */
.keyboard-key.shift-key-uppercase{} /* applies while uppercase */
.keyboard-key.layout-key{}
.keyboard-key.hide-key{}

https://gitlab.gnome.org/GNOME/gnome-shell/issues/46