Improve JAR handling

[Matthias Schoettle]

Currently, a user can load a JAR during runtime and then import implementation classes from that JAR. However, whenever the application is closed and restarted, it has to be specifically loaded again. When generating code, there is also no connection to the JAR.

Although putting this information into the model could work, I think it would be better to do the following:

• When a JAR is loaded, copy it into the concern directory (within a "lib" directory)
• When importing implementation classes, automatically load all JARs within the "lib" directory of the concern (related: #467)
• When generating code, also copy the "lib" directory