Add a hook for registering custom datagrid columns.
DataGrid now has a more formal column registration and defaults setup.

Instead of scanning all columns at instance creation time and populating

some defaults, it now does this once per class. This lets us get rid of

some of our variables and to store less state in memory.
The new DataGridColumnsHook allows extensions to register custom columns

into any DataGrid. This takes care of both registering and unregistering

them.

Added a custom column to the dashboard datagrid. I was able to add it to

my dashboard, move it around, and remove it.