Add panels/ctool content_type plugin for easier display of location data

Now that #487790: Panels integration landed with support for a location context and relationship, we can figure out what kind of location content_type plugin we want to build. Currently, you can do whatever you want using keyword substitution from the location context and a custom content pane.

However, it'd be nice to do something more user-friendly and intuitive for folks configuring panels and location. It could have settings (or perhaps separate plugins) for the following approaches:

A) Respect the node's settings for how to display location fields and just render as if it was a node_view(). This would need some fancy stuff so that the content plugin requires both a location and a node context, and there'd have to be some rules (merlinofchaos says it exists, but it's a bit thorny to get working) to ensure that the location context came from the same node context...

B) Provide checkboxes for which fields to show in that specific pane, but otherwise, format it basically just like a node_view().

C) Provide a text area and the keyword substitution (just like the custom content item does) but perhaps add some value to that in some way (TBD).