containers:selection maps from multiple info.yaml files are not merged

During the config parsing phase, these multiple configurations files can be parsed separately and merge together.

From 5.13 to 5.15 of qtapplicationmanager, configuration handling has been modified drastically.

Config merging works for everything else, but "containers.selection"

Ex.

Below maps of multiple info.yaml should be merged together.

containers:

    selection:

    - some.pattern: "process"

    - "*": "softwarecontainer"

Configurations are merged here.

https://github.com/qt/qtapplicationmanager/blob/5.15/src/main-lib/configuration.cpp#L531

Here only containers.configurations maps are merged.
containers.selection lists must also be merged.

I have attached some sample patches made based on 5.15.0 3276c4feae52ed033be2c2b7b1e15ba3d2dcb470