Handle mouse back/forward buttons

Motivation

While working on !26 (closed) I noticed that my mouse's back and forward buttons behaved like left mouse buttons in Yelp. They did nothing when the mouse was pointed at blank space, and they clicked links when it was hovering over links.

Changes

Now the mouse's back button goes back to the previous page in the history.

Now the mouse's forward button goes forward in the history.

Fixes #6 (closed).

Based on equivalent 8-year-old change from Epiphany:

• https://bug337852.bugzilla-attachments.gnome.org/attachment.cgi?id=231297

... that fixed a 14-year-old issue in Epiphany:

• https://bugzilla.gnome.org/show_bug.cgi?id=337852