Improve lupdate

Merge the QML and the Javascript front-ends. Also, change the
way we process directives in QML/JS comments. The old code
was processing all the directives before visiting the AST nodes.
Unfortunately, this doesn't play nice with TRANSLATOR-like
directives.

Added support for translating extended Javascript files. That is,
QML/JS extends Javascript with pragma directives (e.g. .pragma).
We emulate the `real' QML engine and we simply remove the pragmas
from the original source code.

Change-Id: Ie2536b10199a587b97f41c5e0560acabae981807
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>