[desktoppackage/ConfigurationShortcuts] Use strict === equality in JavaScript

The `property var keySequence` has a value of undefined, empty string,
or a valid QKeySequence gadget type.

Once frameworks/kdeclarative!108 lands, keySequence will be QKeySequence
at all times. Comparing it via === in JavaScript has semantics of
checking operands for being literally the same object (as in: memory
address). I'm not sure how that semantics apply to gadgets which are
supposed to be copied on every occasion, but in the worst case scenario
they'll just always be considered non-equal, and configuration would be
updated every time, which does no harm.

GIT_SILENT