Cleanup

Merge QV4::CompiledData::QmlUnit into QV4::CompiledData::Unit. For pure JS
units it means a slight increase of memory usage by a few bytes, but overall it
makes the code a lot simpler.

Change-Id: Ib48927749720b056f004aac0fe22cb8ec729e3f6
Reviewed-by: Lars Knoll <lars.knoll@digia.com>