Add new PlasmaWindow class

Plasma Window aims to act as an abstract base class for future Plasma purposes to make Dialog more managable.

It provides a window, frame, and matching handling of anything frame related, such as blur and shadows.

Everything else, including positioning, resizing and setting borders will be the responsibility of domain specific subclasses.

Overall goal is to split and fix Plasma::Dialog. This class has been used in a local Krunner branch changing the class "view" inherits from. My proposal is to land this with krunner, then iterate on dialog afterwards rather than trying to do everything at once.