This commit adds the ModelingFeedback component. It hooks into
modeling action rejected events and displays error messages
to the user.
The current behavior is to show an error message when dropping
a flow node outside a participant in a collaboration.
Related to #203
This commit adds the ability to model participants from the palette.
* Empty diagrams can be used as a start for participant _AND_ process diagram
* Process diagrams can be converted to collaboration diagrams by dropping
a participant onto them
Closes#128
