Implement tagset support

[sdruskat]

Atomic should provide tagset support as follows.

• The core plugin should provide
  • An object model for tagsets as per the definition further below
  • A related API for constructing object models for a tagset, from some sort of tagset config file
  • Methods for loading tagsets for a corpus in the editor-related API
  • (Perhaps also tagset-aware default implementations for widgets)

and

• Editor plugins should provide
  • Implementations of tagset-aware editor API (where needed)
  • Logic to provide widgets (comboboxes, etc.) backed by a tagset

In Atomic, a tagset is a definition of annotation values that are valid for a given combination of layers, Salt model elements, namespaces, names and values.

	Layer (SLayer)	Model element (SSpan, SSpanningRelation, etc.)	Namespace (for SAnnotation)	Name (SAnnotation name)	Values (SAnnotation value)
Valid values	List of layers || regex || null	List of elements || null	Empty string || List of namespaces || regex || null	List of names || regex || null	List of values || regex || null
	A list of layers (defined by SLayer names	A list of Salt element types (defined by SALT_TYPE)	A list of namespaces (defined by namepsace Strings)	A list of annotation names (defined by SAnnotation names)	A list of valid annotation values (defined as pairs of value and description, i.e. [value, value description] or [regex, regex decription]