Add Auto-Value Fieldtype

[t0815]

Add a new field type and editor, to configure inserting auto-generated
values.

Need a new EditorType: VariableEditor
Extend Metadata configuration, to allow to configure the target variable string for given field.

Within Metadata-grid, for given field, a button will be displayed. If the user clicks it, the value will
be automatically filled with the given variables content.

Usage: in PageCount, user can autofill the number of pages.

• implement new editor type
• add new configuration dialog
• add new editor to metadata configuration
• handlle variables in Metadatagrid

Editor disabled for now! Does not go towards v1.0