Jasperh/409818 specific color selector

Fixes spinboxes from converting too early when a specific fraction is entered. Beside the improved fractions for Float spinboxes, fractions can now also be used in spinboxes with integers, which solved a visual artifact as side effect.

BUG: 409818

Test Plan

1. Open the Specific Color Selector docker and try multiple different fractions for channels with float depth, as well as integers. For example 1/2, 1/3, 1/4, 1/40 etc.
2. Try other spin boxes (float and integer) and check if they work as expected.

Known Problems

The spin box under Advanced Color Selector > Shade Effect > Line count breaks too early when a fraction is entered. 10/2 for example I tested quite some other spinboxes, but couldn't find any spinboxes where this also occured.

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.