save settings and changes back into the geopackage

[peterhor]

With the plugin we now produce a project file that carries over settings of the geopackage - like symbology, relations etc.
If the file is loaded into a new project, all the settings are not saved within the file, but in the project.
We want the Geopackage to be carrying all this info.

[9ls1]

The default/random colors of the layer "nin polygons" are now saved within the layer in the geopackage file during the project setup.

[peterhor]

The first step (to save symbology) works fine and is resolved in 7e6281c. However, the main challenge remains. It is to save the Attribute form properties of the "nin_polygons" layer within the geopackage (form layout like value-relation based on the tables included in the geopackage, ).