introduce loadFile function for loading JS files

Task-number: QBS-302

Change-Id: I7c36c1ed00905fd3ca47448486a75f0b42108a8a
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
store search paths in file context

This will be needed for loading extensions from JS files.

Change-Id: I1810a5d6afc0427904c2b7502ff4dc91e9507a48
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>