Added Left/Right shortcuts for Timeline increment/decrement.

Also made sure that left/right nudging continues to work for sliders with the focus.

Fixes #52.
Added Slider.set/getPositionIncrement() methods.

This allows the cursor key position nudging behaviour to be controlled.
Prevented Timeline slider focus from masking global frame shortcuts.

The slider increment behaviour was giving us unwanted nudging by pixel amounts without wraparound, whereas the nice global shortcuts give us increments by the frame with wraparound at the start and end of the timeline.
Fixed PathListingWidget cursor key operations.

We need to prevent Qt from passing the cursor keys to our new global shortcut created by the Timeline.
Merge pull request #657 from johnhaddon/frameIncrementDecrement

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