New system for styling configuration

[leendertvanwolfswinkel]

@ldebek before adjusting all styles to the new database schema, I wanted a system of managing the different styles that is a bit easier to manage than the current one.

In this zip, you will find a directory with qml files and a config system: style-v2.zip

The idea is that each layer has n styles, each composed of a combination of qml files (one for each XML key in the QML). For a layer, each style needs to have qml files for the same keys/styling categories. E.g. for weir, in the default styling, there are no labels, but in the "display_name" styling there is a label; so the definition of the default styling also needs to include a qml for the labeling, which will switch the labeling off again when the user switches from the "display_name" styling back to the "default" styling. The StyleConfig class also has a simple is_valid() method to check this; it also checks whether the referenced qml files exist.

I suggest that you find a way to adapt this system to what you think is best and integrate it into the schematisation editor. Than, I will add the rest of the stylings to each of the layers.

[ldebek]

Available for testing on branch: https://github.com/nens/threedi-schematisation-editor/tree/schema_300_styles_v2