Follow-up vector layer symbology

[martinRenou]

As commented in #152 (comment)

Some notes/questions:

- We should remove the "type" entry from the vector layer schema?
- We should follow up on the source-layer property from the schema. We need to follow what QGIS does (be able to edit symbology on a per source-layer basis?)
- What do we do with the opacity property, can we apply the same rules on this?
- We should iterate on the QGZ loader for this
- We should iterate on the Python API for this
- I've seen cases where the symbology panel is not initialized properly when it opens (showing the wrong current render type and value)

We can follow-up on this in separate PRs

• We should remove the "type" entry from the vector layer schema?
• We should follow up on the source-layer property from the schema. We need to follow what QGIS does (be able to edit symbology on a per source-layer basis?)
• What do we do with the opacity property, can we apply the same rules on this?
• We should iterate on the QGZ loader for this
• We should iterate on the Python API for this
• I've seen cases where the symbology panel is not initialized properly when it opens (showing the wrong current render type and value)