Add support for translatable properties
Cambalache currently does not support marking string as translatable or setting translation context and comments.
translatable flags was always in the backend but never used and commit 1bc41ad5 added missing translation_context and translation_comments to object_property and object_layout_property tables (CmbProperty and CmbLayoutProperty wrapper objects)
We need to implement a widget say CmbTranslatableWidget that can be packed in a popover or dialog triggered with a icon on CmbEntry
This Widget need to implement 3 string properties that should be binded to the property wrapper homologous props
'cmb-value' -> 'value' 'cmb-translatable' -> 'translatable' 'cmb-translation-context' -> 'translation-context' 'cmb-translation-comments' -> 'translation-comments'
The wrapper will automatically store all the properties in the DB