Additional text input in printing dialog to be put on the printed page

[mbarto]

We need to implement a configurable plugin for the Printing dialog that will:

• allow the user to insert a text, either in a text input or textarea control (configuration option)
• show a (localized) label to describe the control (configuration option)
• store the text in a custom spec property (configuration option)

This is a very generic plugin, so can be added to the MapStore core codebase.

Notes:

• a custom yaml that includes the additional property is needed for the text to be shown in a printed map.
• more than one instance with different label / property of the plugin should be allowed