Updated documentation for QML Locale.measurementSystem

Added documentation for ImperialUSSystem and ImperialUKSystem.

Change-Id: I1a628121dce4a7566bd8769843c487e28f3dd6fe
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Fix exception thrown in slot without Qml Engine

Don't crash when an exception is thrown in a JS slot but we don't have
a Qml engine.

Change-Id: I1530d5c1c8cb9b9b33b9fdd0d45639fd4a0516f7
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>