Allow usage of mirror tool actions when there's no paintable layer
As shown in https://krita-artists.org/t/greyed-out-mirror-tools-drop-down-options/48535, currently Krita blocks usage of advanced mirror tool actions when the user has a non-paintable layer activated (like group layers). This MR unblocks their usage. I don't see much harm in allowing usage of these actions on nonpaintable layers as they're technically independent of layer usage, and blocking them seems to be one source of confusion.
Test Plan
Make sure nothing breaks when using these advanced actions on the welcome screen or when on a nonpaintable layer such as vector layers.
Formalities Checklist
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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.