Fix Grid server-side header merging and setup (#13334)

This implements simple error handling for broken headers and footers in
client side.

Change-Id: Ic1f1709720fa0b85e5c4c807462a9f9c7eb6f00e
Fix subparts for merged cells (#13334)

Change-Id: Ifa7fc4584cd74d0b3675e6e82a9ad98db8c46d5f
Rename a variable in GridStaticSection (#13334)

Change-Id: I3df6e70a275a9106ae97c8c54319a2b61efb8ca5