Always return a new instance of a Cell

[michalsn]

Description
This PR introduces a change in how we load Cells. Right now we can return either a new instance when we use the full namespace or a shared one when we don't use the FQCN.

This may lead to many problems. This change seems to mitigate all the issues. Also returning a new instance class for every Cell call seems right.

Supersedes #8327
Fixes #8326

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Probably there will be almost no cases where the Cell object must be shared, so this seems to be fine.