Add support for captions to property form items

[toddreed]

• A caption provides additional information about a form item.
• The caption appears in a smaller font than the title.

For example, in a boolean property editor (caption text is the “Morbi leo risus…” text):

To-do:

• Add caption property to RSPropertyFormItem.
• Update RSBooleanPropertyEditor to display caption if provided.
• Update RSFloatPropertyEditor to display caption if provided.
• Update RSObjectPropertyEditor to display caption if provided.
• Update RSPropertyViewer to display caption if provided.
• Update RSTextInputPropertyEditor to display caption if provided.
• Add caption support to RSLabelTableViewCell .
• Add caption support to RSSliderTableViewCell.
• Add caption support to RSSwitchTableViewCell.
• Add caption support to RSTextFieldTableViewCell.