overview: fix issues on the layout CSS

    - fix regresion that broke CSS animation when removing items
    - avoid user agent ul padding to affect grid centering

https://bugzilla.gnome.org/show_bug.cgi?id=724652
Use larger icon for help
Fix really bad typo in help
ephy-snapshot-service: disconnect from "load-failed" signal

Otherwise we have a dangling handler with webviews that are
unrelated to the async result at hand.

https://bugzilla.gnome.org/show_bug.cgi?id=725749
overview: remove unnecessary bottom padding

Remove the extra bottom padding from the overview grid which
was creating an artificial scroll to an empty area.

https://bugzilla.gnome.org/show_bug.cgi?id=725190
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