Show number of open pages on Pages button
Introduce EphyPagesButton which allows to show an arbitrary number on top of a square icon. When the number is more than 100, show an infinity symbol instead.
Forcibly set the label scale to prevent it from changing along with 'gtk-xft-dpi' GtkSettings property.
Use EphyPagesButton instead of the current pages button, and sync the shown number with the number of open tabs.
Partially based on !279 (closed)