This allows the viewer to use either PyQt4 or PySide as the toolkit.
Avoid use of matplotlib's pyplot functions for creating figure canvases. In particular, key press events were ignored by the canvas.
I saved a copy of the line profile plugin in a branch and then deleted the plugin from the main qtmpl-viewer branch. Unfortunately, I forgot that rebasing off the main branch would erase the plugin. This commit just adds the plugin back.
Tony S Yu