l10n: Use Qt's own lconvert to produce .pot files out of the .ts

l10n: Use Qt's own lconvert to produce .pot files out of the .ts

The Qt's documentation [1] says that ts2po is known to cause trouble. We've seen
that (see Chusslove's patch from yesterday), and even after his fix, the
produced .pot file still doesn't contain proper file location information. Using
lconvert shall do the trick.

Sorry for those localization teams which have already done the work, this is
pretty much a new area for me.

[1] http://qt-project.org/wiki/Qt-Localization