ephy-window: fix shift+click

The check for the clicked address was inverted so this was not working
at all.

https://bugzilla.gnome.org/show_bug.cgi?id=725818
Remove redundant comparison

https://bugzilla.gnome.org/show_bug.cgi?id=725818
ephy-window: move event state and button checks earlier

Micro-optimization

https://bugzilla.gnome.org/show_bug.cgi?id=725818