Create Madx instance with every new frame

There is a subtle but major bug in this change:

NotebookFrame.Reserve creates new frames with models that are based on
the Madx instance of the initiating frame.
Add method NotebookFrame.Claim

The method is used to transfer ownership of the frame to a specific
model/file. This fixes the bug mentioned in the previous commit message.
Set help messages for 'Open' menu items  dynamically
Merge pull request #37 from coldfix/frame-equals-madx

Immediately start a Madx instance for every new Frame