mainWindow: Convert zoom bindings to accelerators

Adding a accelerator instead of key-press-event,
Which allows user to perform operation without mouse.

https://bugzilla.gnome.org/show_bug.cgi?id=731540
Add accelerator to grab focus on search

Ctrl+F is added as an accelerator to grab focus
on search entry.

https://bugzilla.gnome.org/show_bug.cgi?id=731540