Fixed folding/unfolding when clicking thumbnail, traversing layers, modifying layer tree

Fixing BUG:408204

What's changed:

1. Reverted commit c475619e to restore automatic item unfolding on creating child node, folding/unfolding on thumbnail click (with no modifiers).
2. Returning false in case of clicking thumbnail with no modifier key pressed to avoid unexpected layer selection on canvas.

Now, if you don't use modifiers it works exactly as in 4.1.7, in exception when you click thumbnail, the layer item in the three gets selected (the selection on canvas keeps unchanged). I believe it's more natural behavior than select it only when modifier keys are pressed.

Of course if you click thumbnail when pressing modifiers, the new canvas-layer-selection behavior is working.

Test Plan

1. Check folding/unfolding of layers with masks and groups on thumbnail click
2. Check automatic unfolding the layer when creating layer mask, surrounding the selection by the group, traversing the tree by Page Up and Page Down

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.