Draft: Move docker action submenu to Window, add docker menu toolbar action

Inspired by https://github.com/LainFenrir/krita-resource-docker-menus, this commit moves the docker-related actions from the Settings menu to its own Dockers menu, for better discoverabiliy and usability.

TODO:

• Move the Dockers submenu from Settings to the Window menu instead.
• Add a toolbar action that would act as a menu for activating dockers.

Test Plan

Test various Docker actions in the Dockers menu.

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.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.