Bug 390952 : Add an option to snap to pixels

Add an option to snap to pixels

Test Plan

• Open Krita
• Make a vector layer
• Create a shape
• Switch to Edit shapes tool
• Switch to Pixel Snapping mode (use Shift + S menu)
• Try moving any of the anchors
• The anchor would snap to the pixels

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.