Install windowpicker applet from Debian

As discussed on #7349 (closed), we’re shipping a very crude (to put it mildly) window-picker-applet package. This code has since been merged into the gnome-applets repo. We’d like to replace our current package with the one from Debian, once there’s a new upstream gnome-applets release that makes its way into jessie-backports.

Subtasks

• #8113 (closed)
• #8112 (closed)
• #8111 (closed)

Related issues

• Related to #7349 (closed)
• Related to #8064 (closed)

Original created by @intrigeri on 8147 (Redmine)