Tweak various strings belonging to fill/selection tools

This MR tweaks various strings belonging to the fill/selection tools. Notable tweaks include:

• "Multiple fill" option is renamed to "Drag fill behavior" "Drag-fill mode" to better convey the functionality and its usage to users that might not be familiar with the feature.
• Enclose and fill tool's "What to fill" option is renamed to "Target regions", and its item strings have been tweaked, in order to remove redundancies so that the interface is more concise.

Before:

After:

• "Fill with" option is renamed to "Fill source" as I just believe it sounds better.
• "Reference" option is renamed to "Reference layers" to better convey its usage without relying on tooltips, and to better differentiate its potential meaning from "Target regions" and "Fill source".

This should probably be reviewed by @deiflou as I think he's responsible for these UIs in general.

Test Plan

Observe new strings in fill tool, enclose-and-fill tool, and contiguous/similar color selection tools.

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.