Make dock windows resizable but do not allow tiling

Luca Carlon requested to merge lcarlon/kwin:resize_dialog into master

This is a proposal to support MR frameworks/plasma-framework!500. The final target is implementing what is requested in

With this patch kwin should make docked windows in wayland resizable, while preventing the user from tiling. MR frameworks/plasma-framework!500 would then be able to startSystemResize even when the docked window is frameless.

A demo of the final result:

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