Added Left/Right shortcuts for Timeline frame increment/decrement.

This fixes #52, and I think means we can close pull request #559. I've implemented this as a window level shortcut present whenever a Timeline is in the layout, rather than as the menu items Ben had in #559. I believe he only made the menu because it was the only way he had of making a shortcut, rather than because he wanted one. Also, in our cache viewing use case, the main menu doesn't exist so that approach wouldn't work there anyway.