add value setter to represent an existing signature

[gullerya]

Since standardization called to maintain value property:

• getter is already well defined in another place
• setter should be added here:
  • set to null should effectively clear the signature (another ticket mention removal of clear method)
  • set should also accept string that will be treated as outerHTML of SVG element
  • still not clear if set should also accept Blob that will be treated as canvas input
  • in any case - when value is set with an existing signature (not empty) - element moves into a read-only mode and only clearing it 'unlocks' that (or if explicitly 'unlocked' - do clear)