Use Qt6 shadertools from within Qt Design Studio

When vertex and fragment shader files are edited, Qt Design Studio picks the change and tries to apply them to Shadereffect - BUT it does not work in Qt6 as Qt6 uses binary shader format.

Existing toolchain needs to be changed so, that when Qt Design Studio sees frag or vert shader saved, it needs to trigger 'qbs' binary and compile the shaders.

Resulting artifact is stored in the same directory where shader source was.

If there are errors, 'qsb' tool directs to stderr so they can be shown in Qt Design Studio's output pane.