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Instead of two RadioFields we now use one row of multiple Checkbox field

this allows to be more flexible for the container and its associated columns, is easier to program and understand
Using new data layout for glossary['breakpoints']
refactored reduce_breakpoints into utils
whitespace fixes
Fixed typo
media queries are computed while editing BootstrapContainer
Extracted some methods from PicturePlugin to be reusable for SlidePlugin
Simplified code
Removed useless import
Enforce glossary_fields to be a list or tuple