Animation: KisNodeManager Added Sibling Selection Actions.

Added a couple of new actions which allow users to navigate the node tree without entering groups. Found under Actions>Krita>Layers>"Activate next/previous sibling layer", in the shortcuts section of the configuration menu.

Feature requested by Bollebib.

Test Plan

Bind keyboard shortcuts to "activate previous sibling layer" and "activate next sibling layer". (I recommend Alt+PageUP/PageDown, but there is no default.) Try to navigate a Krita document with lots of layers and groups. When using these actions, only nodes on the same level of the tree should be selected, and groups should not be entered or exited.

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.