Settings: auto-load manifests

[inercia]

Auto-load manifests in some user-provided directory.

[inercia]

This would require:

• to create a new setting for the directory for these manifests
• to add some extra text box in either
  a) one of the Preferences tabs
  b) one of the tabs in the New Cluster window.
  and bind the textbox to the setting we previously created.
• mount the manifests in the same way we currently do with charts (but instead of using self.charts use something like self.manifests)
• create a function that reads all the manifests in the directory pointed by the setting.
• pass the manifests obtained with this function in the same way we do with charts.