Storyboard undo/redo

Implementation of undo/redo for storyboard addition and deletion.

These are the changes in this MR :

• Renamed slotInsertChildRows to insertChildRows and call it explicitly whenever a first level node is added.
• Added a TotalDurationInFrames and TotalDurationInSeconds data roles as this functionality was used in quite a lot of places.
• Changed the removeRows function so that the deleted scenes duration will always get added to the previous scene. Previously this didn't happen with the last scene.
• Changed the lastKeyframeWithin(QModelIndex) function to use the next scene's frame number whenever possible.
• Disconnected unused signals for keyframe addition and deletion.
• setData return false for duration fields if implicitSceneDuration is equal to total duration in frames for the scene.
• Implemented the command classes that inherits from KUndo2Command and added proper commands to stack whenever scenes are added, deleted or moved or when scene duration is changed.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.