feat: external packages can be loaded w/o registering them in app.json

[ThorstenSuckow]

Ext.manifest should be updated once the application configuration with registered packages was parsed.
Packages should be dynamically loaded based on

• their appearance as package in the uses-section of app.json
• their availability as Sencha cmd-package in the appropriate directory of a build

If the application configuration has entries which do not appear in Ext.manifest, Ext.manifest should get updated with this packages. The package should then be loaded.