Add an explicit layer docker menu button for showing the context menu actions

BUG:446649

This commit adds a menu button to the Layer Docker's Properties button which will grant the same layer operation actions as the right-click context menu. This is so that users without access to buttons on their stylus can now use this menu and some of its exclusive actions (outside of shortcuts).

Although this commit technically doesn't add new strings, it moves old strings to a new function, so I'm not sure if it's backportable.

Test Plan

Check if the right-click context menu and the layer operations menu button show all their actions properly. Check if they update their actions corresponding to the current state of what's selected in the layer stack. Check if the actions actually function.

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.