Element content refactoring

[brandonkelly]

• All fields should be stored globally (remove the context column from craft_fields)
• Each field layout should correlate to a “field set”. Each field set has its own content table (craft_content_X).
• Field sets can be localizable or not, determining whether their content table has a siteId column, and whether that should be factored into element queries.
• Field sets can override Name, Handle, and Instructions settings, and possibly some field type-specific settings (e.g. the "Add button label" in relational fields).
• Element Query classes should be able to specify the field set IDs that will be relevant to a given query.
• Each element should store its field set ID, so Element Queries know which field set the element is associated with (which custom field columns' values to care about) just by looking at its row data.
• Element titles should be moved to the craft_elements_i18n table, next to Slugs.
• Field Layout Designers should have inline field creation, to make up for the fact that Matrix field configurators will just have users select fields for their block types, rather than creating them.

[j-greig]

Excited for this one!