Make "color_by_attribute" of circle_layer work neatly when activate layer

[roeldegoede]

While switching tabs in the GUI, layers are hidden/shown and activated/deactivated. The color_by_attribute feature gets lost when using activating the layer, which means that the circles are not coloured anymore, whereas the data is still the same.

By calling "update" after "activate", this issue is solved temporarily, see this commit

Would be good to check the code and make the activate function work immediately.

Example of colored dots (a geodataframe with Point geometries colored by their name):