Allow using guides for the imagesplit dialog.

The image split dialog currently can only do splits at intermittent intervals. This patch enables the ability to use the horizontal and vertical guides for this. This is basically what the gimp guillotine plugin does as well, though with 100% more french-revolution-reenactment that is out of scope to Krita.

Test Plan

Open an image add some horizontal and or vertical guides and go to image->image split where you should toggle 'use guides' on the relevant guides.

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.