Add feature for splitting layer into local selection masks based on color

The feature request is here. This feature will add a comboBox in Layer>Split>Split_Layer dialog which will have option to either split layer into layers or local selection masks. Layer will be split in layers or local selection masks accordingly. This will allow easy blending of colors.

Test Plan

• Go to Layer > Split > Split Layer...
• See a comboBox at the top of the dialog box. It has two options Split into layers and split into local selection masks. Previously their was no comboBox and splitting into selection masks was not available.
• Choose Selection masks and see that option not relevant for selection masks are hidden.
• Click Apply and see that the layer is now split into selection masks based on colors.
• Also the progress bar shows Split into masks and split into layers where relevant.

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. /label Needs Review