Delete custom kpToolAction class
This commit has no user-visible effect. This commit deletes the kpToolAction class which is a KolourPaint- specific subclass of KToggleAction. kpToolAction used to have the specialised functionality of creating a custom tooltip that showed the keyboard shortcut but this functionality was removed in the previous commit because it became redundant thanks to a frameworks change. Because of this there is no reason left to keep this subclass around. Instead its base class KToggleAction is used everywhere.