Changed up the tree structure.

Notable widgets/ is now in render/ along with core/text.* and core/utility.*.
Updated the request formatter.

There were a couple of semantic errors that were legal syntax but made
no sense once instantiated.
The pong::Response structure is no longer a template.
Updated server/plugins.* under core/plugin/plugins.*.
Subdirectories of core/ now each have their own CMakeLists.txt file.
Request names now use underscores between words.
Server_Plugin is now Plugin.
Fixed up some semantic errors in pif and the Request variant formatter.
New dumper formatter definition for a pong::Severity.

TODO: Make some macro to easily define formatters for enumerations.
New request which exits the engine.
Implemented the exit request in the engine + A basic plugin that uses that request.