Change the title bar of the toolbox back to the previous one

Reverted the titlebar widget of the toolbox back to the previous one. The current one breaks visually when the ToolBox doesn't have enough width. Also the fact that some people would start complaining for the Tool Box after they close it accidentally, which if I am not wrong was the reason that the ToolBox used a plain Frame.

Test Plan

Build and start krita, you would get the old title bar for the toolbox docker.

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.