add confirmation dialog for 3rd party fragments

[chrisrueger]

• show a confirmation dialog when at least one of the templates fragments to be installed is not from bndtools github organisation.

• this is similar to the dialog shown in bndtools NewWorkspaceWizard with fragments
• this increases security as user consent is required for installation of potentially untrusted templates

Example

You have selected 1 fragments from 3rd-party authors: 
IndexedMavenRepository - Provide an OSGi repository derived from a subset of one or more maven repositories
  - Organisation: mnlipp
  - Repo: https://github.com/mnlipp/de.mnl.osgi/tree/master/de.mnl.osgi.bnd.repository/workspace-template 

Are you sure you trust the authors and want to continue fetching the content? (yes/no): y
Added 2 template fragment(s): 
osgi - OSGi R8 with Felix distribution
  - Organisation: bndtools
  - Repo: https://github.com/bndtools/workspace-templates/tree/master/osgi 
IndexedMavenRepository - Provide an OSGi repository derived from a subset of one or more maven repositories
  - Organisation: mnlipp
  - Repo: https://github.com/mnlipp/de.mnl.osgi/tree/master/de.mnl.osgi.bnd.repository/workspace-template