Better Dependency Checking

When OpenShot loads up, we need to have a more elaborate dependency check. Right now we have no error handling around our import statements, and thus... OpenShot will just break if you are missing a library. We need to gracefully tell the user to install the missing dependencies, instead of un-handled errors.

Check for these libraries:
mlt, gtk, goocanvas

For example:

try:
   import mlt
except:
   # Show friendly error "Sorry, but you need to install MLT"