Port Qt code to new container assign() methods

See epic for more details.

Find candidate assign() users and port them to the new function.

E.g.

• readInto()-like functions
• functions that contain a loop and re-populate a Qt container in every iteration

This is intentionally fuzzily worded, but we mustn't just add new API without looking around our own code base for potential users, and port them.

Acceptance criteria:

• at least one user in Qt production (non-test, non-example) code for each of QString, QByteArray, QList
• (if possible) at least three relevant users generated in each of QtBase and QtDeclarative modules
  • "Relevant" means: saves actual memory allocations compared to the old code
• advertize the commits on the development ML to educate fellow Qt devs