TableView: fix a performance issue with row delegate instantiation

To avoid accidentally instantiating all rows at construction time, make
sure the rows have a valid (but arbitrary) height meanwhile the row
delegates haven't yet been loaded.

Change-Id: I1afc6bfa920886260717b3cb172fa0d858c0a8d5
Reviewed-by: Caroline Chao <caroline.chao@digia.com>