support internal extensions in loadExtension

Internal extensions like qbs.File are now supported by loadExtension.

Task-number: QBS-302

Change-Id: I2b51fec90145afbe18f48cfdd5c0becc9472e90c
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
load PropertyList extension in bundle-tools.js

Task-number: QBS-549
Change-Id: I1a02ec28aa1be4191b8e6070674b3407154cfc98
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>