layerstack: Added moveToSelectedToRow()
ShortcutManager: Instead of warning the user about duplicate shortcuts, simply disable conflicting shortcuts.
Newly registered shortcuts take precedence over previously registered shortcuts.
BrusingController: Don't draw a brush stroke if the user double-clicks.
(Handle single clicks via a QTimer that is cancelled by double-clicks.)
LayerWidget: Draw alternating row background colors.
tiling: 'direct' layers are not 'direct' for prefetching requests.
Navigation Controller: Scroll N slices at a time depending on the speed of the scrool wheel.