effects: add Meta+plus as a default shortcut for Zoom effect
Right now the Zoom effect's "zoom in" action is bound by Meta+Equals, which makes sense for ANSI US keyboards without numberpads which make you hit the Shift key before you can get a plus sign, and pressing it without Shift gets you an Equals sign.
But some ANSI US keyboards have a numberpad with a Plus key, and many other keyboards without numberpads also have a Plus key, so we should allow users of those keyboards to trigger "zoom in" as easily as users of ANSI US keyboards.