Current map scale shown in printing dialog and printed page

[mbarto]

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

• show the current printing scale with a checkbox to include/exclude the scale from the final pdf
• show a (localized) label to describe the control
• store the user choice in a custom spec property (configurable option)

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

Note: a custom yaml that includes the additional property is needed for the scale to be shown in a printed map.

[mbarto]

Depends on geosolutions-it/MapStore2#7595