Print extent widget

[mbarto]

We need to create a widget like the one used here: http://maps.cmparks.net/geoserver/www/printing/index.html (click on the print on/off button).

This should appear / disappear together with the print dialog and allow to:

• pan the print extent
• resize the print extent (constrained to the available scales)

The extent should be synchronized with the print panel scales combo and chosen layout (to fix the aspect ratio)

The component should be part of MapStore2 standard components and be enabled by a flag in the Print plugin

[mbarto]

Continue the work from https://github.com/geosolutions-it/cmp-webgis/pull/20