Extend the QML bootstrap library by the IR builders

This is among other things needed to fix the qml import scanner to detect
dependencies from .js files correctly.

The patch also fixes the use of Q_QML_EXPORT towards Q_QML_PRIVATE_EXPORT
where appropriate and corrects the wrong include path for the double conversion
code to actually be relative to the file it is included from. This worked by
accident because of other include paths present in the build.

Change-Id: I338583dad2f76300819af8ab0dae8e5724c84430
Reviewed-by: Lars Knoll <lars.knoll@digia.com>