Not touching the Polish files since they appear to be up-to-date (the
scripts also produced changes for these files but they don't seem very
meaningful; I can of course send the resulting files to Roman if he
wants to inspect the changes).
This brings a developer comment to the FlightGear-Qt.xlf files that will
hopefully draw the translators' attention on the fact that they should
put the *up-to-date* address of the "getstart" manual in their language
(use correct address + two occurrences of 'en' to change).
Remove no more used `menu/interface-config`.
Add `menu/highlighting`, `menu/vr-options`, `options/system-time`,
`options/gmt`, `options/local-aircraft-time`.
Correct tip text from "Hold [Shift] while looking around to move the
view up, down, left or right" to "Hold Ctrl ...".
Corrections for Polish nonQt translations.
