checkwps: Validate translated strings (%Sx)

In verbose mode it will log valid the strings found, otherwise it will
only complain when we encounter a missing string.

Unfortunately a missing string is not inherently a problem, due to
conditional expressions.  So all we can do is complain in checkwps
or if wps debugging is turned on.

Meanwhile, this is the first step in actually enumerating the translated
strings used by themes.

Change-Id: Ia93b333085e825d5b085c4d372ad8e13aa3e3ba1
9 files changed