Cherry-pick MR for !462.
Previously, dragging the slider on one instance of the applet wouldleave other instances untouched and thus out of sync. The sendingapplet instance would include a hardcoded source client contextstring in its setBrightness D-Bus method call, but all instanceswould ignore the resulting brightness change event with this context.
To fix this, make sure that each applet instance uses a differentcontext string. We'll increment a number in each constructor anduse it as part of a context string template. Only the sending appletinstance will ignore brightness change events caused by its ownbrightness setter call.
(cherry picked from commit 5a06866f)