Add hover feedback for tab close button

Adds hover feedback (a red background) for the tab close button.

simplescreenrecorder-2021-06-15_22.33.53.mkv

Test Plan

Check for hover feedback on the tab close button.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• n/a 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.