[Map-Printing][Map] API for Map Export State

[mbeckem]

Currently, the printing package creates a "screenshot" of the open layers map to print it (using the html2canvas library).

Some UI elements (such as help tooltips, or draw indicators) should not be shown when creating the screenshot.

Currently, two methods are used by the printing package:

• It provides a css class (printing-hide) that can be attached to arbitrary DOM-Elements. These (and their children) will be ignored during screenshot creation. This approach is good and can be kept.
• It manually hides active draw interactions to get rid of the indicator. This is not future proof (or well maintainable) and should be replaced by a better system (see below).

The idea is as follows:

• When an interaction (selection, measurement, ...) is created, it registers itself on the map model (or some sub-interface)
• The registration includes callback methods to destroy and to temporarily disable the interaction
• When the printing package (or some other package) wants to print the map, it temporarily disables interactions (which in turn hide the appropriate screen elements) and enables them again later