Consolidate/refactor 'gridrow_nav' with REST support - fixes #34

Newer (comparatively) grid/datastore REST NAV code has been refactored
and consolidated into the 'Ext.ux.RapidApp.AppTab.gridrow_nav' function,
which is the original nav handler prior to the Rest (hashnav)
conversion. It now detects REST support and uses it by default, but
still falls back to the original 'loadTarget' method.

Now, everything go through this codepath, including
autoload_added_record, which further hides the non-REST urls. Now just
about everything by default should be using REST urls.

This code is still ugly/tangled, and needs a larger refactor and
elimination of the old loadTarget/non-REST APIs altogether. This will be
a lot of work and lots of possible ramifications, however, and has been
put off for a planned refactor for RapidApp 2 down the road. The code is
stable, just a bit harder to understand/maintain currently...