ImageField: support image as Pdata
display attachment's filename and content_type in Event view.

now it is filename, not file_name.
When installing BT, update local roles of path items at the end of the transaction

Updating local roles may depend on many things, like categories, and developers
should not be forced to put all dependencies in a separate BT that would be
installed before.
ActivityTool: Add some useful tips
Update Cached Convertable interface

- The interface says that getConversionSize raises a KeyError when this cache
  does not exist. Update implementation accordingly.
- update getConversion to always return a string for the data. The caller
  should not have to worry about the data format.
- warn when an OFS.Image is passed to setConversion