I've added a keybinding for switching focus to the split window editor (in case it's enabled). This solves #631
I've set F3 as keybinding by default because:
1. Follows the idea of the other "Focus Keybindings" family: F2 -> Switch to Editor, F4 -> Terminal, F7 -> Search bar
2. Doesn't collides with any other default keybindings and has very few possibilities of collision with user-defined keybindings.
3. Even in the case it collides with other user-defined keybinding, it's not a big deal since it just grab the focus on split window only in the case it's being displayed. If user doesn't like it, he/she can always change this.
4. It's a plugin shipped along with geany.
