Issue #1851018 by alexpott, attiks, xjm: Improve breakpoint configuration implementation.

core/includes/theme.inc

core/modules/breakpoint/breakpoint.install

core/modules/breakpoint/breakpoint.module

core/modules/breakpoint/config/breakpoint.settings.yml

core/modules/breakpoint/config/schema/breakpoint.schema.yml

...kpoint/lib/Drupal/breakpoint/BreakpointGroupInterface.php

...eakpoint/lib/Drupal/breakpoint/Entity/BreakpointGroup.php

...t/lib/Drupal/breakpoint/Tests/BreakpointGroupCRUDTest.php

...t/lib/Drupal/breakpoint/Tests/BreakpointGroupTestBase.php

...point/lib/Drupal/breakpoint/Tests/BreakpointThemeTest.php

.../tests/config/breakpoint_theme_test.breakpoint_groups.yml

...akpoint.breakpoint.theme.breakpoint_test_theme.mobile.yml

...akpoint.breakpoint.theme.breakpoint_test_theme.narrow.yml

.../breakpoint.breakpoint.theme.breakpoint_test_theme.tv.yml

...reakpoint.breakpoint.theme.breakpoint_test_theme.wide.yml

...oup.theme.breakpoint_test_theme.breakpoint_test_theme.yml

...int.breakpoint_group.theme.breakpoint_test_theme.test.yml

..._theme/config/breakpoint_test_theme.breakpoint_groups.yml

...t_test_theme/config/breakpoint_test_theme.breakpoints.yml

...ules/picture/lib/Drupal/picture/Entity/PictureMapping.php