The Imagine style is based on image assets. The style comes with a
default set of images, but the images can be easily changed by
providing a directory with images using a predefined naming convention.

[ChangeLog][Controls] Added the Imagine style, which is based on
image assets that can be provided using a predefined naming convention.

Task-number: QTPM-517
Change-Id: I550d7dac9a9686d60bec15655ac92dea9f36149c
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>