magnifier.js: fix source id warning
notifications: fix default opacity being doubled in hidpi mode.
desklets: Make sure a new desklet is placed in a visible portion of the
primary monitor.

Fixes: #3724
photo desklet: Fix a warning, clock desklet: move some String class overrides
to js/ui/overrides.js so they can be used in the desklet as well as the
calendar applet.
panel applet: always remove the drag placeholder after a drag (not
just after a successful one)
menu applet: always remove a favorites drag placeholder after a drag
(not just after a successful one)