Further straighten out alias compilation

QQmlComponentAndAliasResolver<QQmlTypeCompiler>::resolveAliasesInObject) currently amends the Aliases in the QML IR. This is somewhat bad style. We should

1. Populate aliases already at compile time with all information available then
2. Move information not available at compile time into the property cache.

The bits left here are:

1. targetObjectId
2. isAliasToLocalAlias
3. the AliasPointsToPointerObject flag (do we actually need it?)
4. localAliasIndex
5. idIndex

It's a good idea to compare with the implementation of QQmlComponentAndAliasResolver for QV4::CompiledData::CompilationUnit since that one only has const Aliases anyway and needs to do the same work.