WIP: Add actions for zoom fit to page, width, and height

Currently Krita has canvas input shortcuts for fit to page, width, and height, but they aren't currently as accessible as native actions. There is an existing "Toggle Zoom Fit to Page" action, but since it's a soft toggle, it can give unexpected results.

This commit adds native actions for Zoom Fit to Page zoom_to_fit, Zoom Fit to Width zoom_to_fit_width, and Zoom Fit to Height zoom_to_fit_height, and as such they'll be now accessible through the menu, toolbars, command bar, and scripting.

TODO:

• Move the canvas input default binds over to the corresponding actions

Test Plan

Test the three different actions though some variety of canvas positions/sizes, through various different means.

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.