changed shader variables peerInside and difftrans

(diffuse translucency - or forward scattered translucency) to boolean
variables. renamed sliders to
shininessSlider/opacitySlider/darknessSlider. added png reader to read
in background image for main vtk render window, set depth peeling
occlusion ratio to 10.0, renamed light toggle button to btnLight,
created setSelectedMesh method that auto updates list, sliders, sets all
selected shader variables to false except for selected mesh.

thetaroundness and phiroundness are created using slider parameters (and
chkToroid checkbox), depth peeling seems to work (Without wiggle). Light
toggle changes icon on click (loaded from resource). Listbox now has
alternating colours and list items can be dragged around and reordered
in the list. added bg.png; interactorstyle now calls public slots from
mainwndow. Figure out slowdown