TextSymbolizer label placement

[jansule]

Feature Request

Is your feature request related to a problem? Please describe.

The TextSymbolizer does currently not support a property for setting the label placement. Due to this, we are currently not able to make the labels of a line (e.g. a street) follow the line.

Describe the solution you'd like

We should add a property (e.g. labelPlacement) that handles this.

In mapbox this property is called symbol-placement (see https://docs.mapbox.com/style-spec/reference/layers/#layout-symbol-symbol-placement).

In OpenLayer this property is called placement (see https://openlayers.org/en/v7.5.2/apidoc/module-ol_style_Text.html#~TextPlacement).

Describe alternatives you've considered

None

Additional context

Style using different placements due to lack of support:

[KaiVolland]

It currently also extends the BasePointSymbolizer which is incorrect as it won't fit the needs for line string labeling. The TextSymbolizer already contains line based properties like maxAngle so i guess its no problem to move the matching properties from the BasePointSymbolizer to the TextSymbolizer and add the placement property.

[jansule]

Partially solved by #584

[KaiVolland]

The TextSymbolizer still extends BasePointSymbolizer which is not correct. This should be fixed as described here:

It currently also extends the BasePointSymbolizer which is incorrect as it won't fit the needs for line string labeling. The TextSymbolizer already contains line based properties like maxAngle so i guess its no problem to move the matching properties from the BasePointSymbolizer to the TextSymbolizer and add the placement property.