Allow printing grids with labels

[mbarto]

Using #35, we want to implement a printing dialog control to include grids with labels in a printed map.

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

• show a checkbox to include/exclude the grids from the final pdf (using the chosen CRS to decide which kind of grid)
• show a (localized) label to describe the control
• store the user choice in a custom spec property (configurable option)
• implement a custom mapfish payload transformer to include a e grid layer when the checkbox is checked