Default alt and title text with token support

I've created a patch that adds support for setting custom default alt and title text with support for tokens.
Since ImageField is commonly used in a clearly defined context it can be useful to use data from the node in a default alt and title text. A typical use case would be for a Product node type on an e-commerce site where the node title can be used as the product shot alt text. This obviously also removes the burden of entering custom text from the user (and the burden of educating end users on the importance of alt text from web developers).
This could also be useful as a SEO tool.

This is somewhat related to #193887: Accessibility enhancements.