Grid agOptions prop is not reactive

[ghsolomon]

Currently, agOptions provided to aGrid component via props are read and cached when the component first mounts. This is unintuitive, since typically components react to observable prop changes.

For example, in a client-app, users are presented with an input to toggle the domLayout prop between normal and autoHeight; however, in order to actually achieve this, theGrid needs to be remounted to force the agOptions to be read again.

A couple possible solution:

1. Stop caching agOptions so they are reactive like most other props. (We could do this by making agOptions a computed getter on GridLocalModel.)
2. Make agOptions a readonly property of GridModel to more clearly communicate its immutability (does not solve the application issue described above)