Match[parent_class] - new matching way to override templates

*Title: Match[parent_class] - new matching way to override templates.

*Documentation:
New match condition is implemented to override templates by content class
of the parent node for the current viewed node for 'view' function of 'content'
You can set it using any of two new matching keys:
- 'parent_class' used to match by ID of the content class of the parent node;
- 'parent_class_identifier' used to match by class identifier of the content class of the parent node.

*Example:

# these two override sections are identical:
# matching by parent_class
[article_override_1]
Source=node/view/full.tpl
MatchFile=your_template_1.tpl
Subdir=templates
Match[parent_class]=1
Match[parent_class_identifier]=folder

# or matching by parent_class_identifier
[article_override_2]
Source=node/view/full.tpl
MatchFile=your_template_2.tpl
Subdir=templates
Match[parent_class_identifier]=folder