Add a translator as a fallback to Qt inner one

Add a translator as a fallback to Qt inner one

This reuses code intended only for mac manus and generalizes it. The
list of strings to add to po files is found in
GuiTranslator::translate.

This is useful now that LyX relies on QDialogButtonBox class for its
dialogs. Indeed many languages are not covered natively by Qt.

It is possible to enable the "locace" debug channel to see what
strings are not covered and should be added to our own translation
tables.

In order to make things easier, a new method getIfFound() has been
added to the Messages class, which returns an empty string when no
translation has been found, as Qt's translate() does.