Transform content based on feature.contentTransformations.
Add basic content transformations for image and table plugins.
Simplified contentTransformations.
Prevent conflicts where running filter fn twice at the same time.
Made transformations more accurate - only first one in a group which is matching ACRs will be executed.
Implement content forms unification by filter.
Unprotect and then protect back elements names.
Docs for new features.
Apply transformations when checking content.
Fixed typo.
Merge branch 't/9989' into major
Introduced ACRs with required properties.

* Styles/classes/attrs starting with '!' are required. Element without
them won't be accepted by this rule.
* Required styles/classes/attrs may also be set by requiredStyles/requiredClasses/requiredAttributes (string/array).
* Name of rule.validate has been changed to rule.match which makes more sense. Now its meaning
is simpler - rule has to match to element to be applied to it.
Use ACRs with required properties.
Made all CKEDITOR.style's parts required.
Merge branch 't/10006' into major
Remember names of features in ACRs registered by them.
Set command name on definition - like for UI definitions.
Pass command, not command definition to editor#addFeature().
Merge branch 't/9994' into major
Basic image dialog integration with ACF.