Clean up diagramscene example's toolbutton icons.

Clean up diagramscene example's toolbutton icons.

Remove artefacts from pointer.png.

Replace several PNGs with images without artefacts (used FreeSerif as
the font).

Stopped scaling images up in MainWindow::createColorToolButtonIcon.

Change-Id: I7adf2deea73b89c631d39d575804e8d3f58c9fe1
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>