gui.plugins.contenthandler: allow to visualize different background images using a slider

git-svn-id: svn+ssh://svn.gna.org/svn/advene/trunk@7625 3fb6f576-06d4-0310-ad11-a4b695aa9f05
Update INSTALL.txt instructions

git-svn-id: svn+ssh://svn.gna.org/svn/advene/trunk@7626 3fb6f576-06d4-0310-ad11-a4b695aa9f05
doc/TODO: update

git-svn-id: svn+ssh://svn.gna.org/svn/advene/trunk@7627 3fb6f576-06d4-0310-ad11-a4b695aa9f05
gui.edit.shapewidget: fix validate test

git-svn-id: svn+ssh://svn.gna.org/svn/advene/trunk@7628 3fb6f576-06d4-0310-ad11-a4b695aa9f05