should validate location information from ts files

linguist's "sources and forms" window should verify that the ts file is newer than the source files it refers to, as otherwise the source location information may be outdated.

taken further, linguist should rescan the sources on the fly, making the line number information in the files superfluous in the first place. the ts files would merely need to contain the command line and working directory of the lupdate call which created them (or equivalent information).