Fix export functionality

Problem/Motivation

Exporting breakpoint groups with features leads to some inconsistent output.
There are notices about missing variables and the relation between breakpoint groups and breakpoints isn't exported at all.
Further the export in the UI heavily differs from that in features.

Proposed resolution

Add breakpoints_breakpoint_group_export() as export callback to the schema definition of breakpoint groups.
Refactor breakpoints_breakpoint_group_export() to only export the machine names and not the whole class of the related breakpoints. Further adjust the output to respect the coding standard as well as respect the indent of the export context .
Modify breakpoints_admin_breakpoint_group_export_form() to display dedicated textareas for the breakpoint group export and the related breakpoints.

Remaining tasks

All those changes are done in the attached patch.
Review needed, I'm not sure if I covered all export scenarios.

User interface changes

The form for the breakpoint group export (admin/config/media/breakpoints/groups/%group/export) contains two textareas now. One for the breakpoint group and one for the related breakpoints.

API changes

none