dual color selector shortcut

Adds two shortcuts that open the same foreground/background color selector as Clicking the squares of the dual color widget in the toolbar.

Requested on KA here

Test Plan

Choose a shortcut key for the "Open Foreground color selector" and "Open Background color selector" and use the shortcut

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.