Add styling for output layer of main plugin

[merydian]

Basic blue line from QGis default styles.

[MichaelsJP]

@merydian What about taking the colour we also use in our maps client?
Seems a bit more aligned.

[merydian]

Should we add the start and endpoint markers as well?

[koebi]

Could you add a screenshot with start- and endpoint-markers, and the current style? That would make it easier to reason about :)
Tagging @TheGreatRefrigerator here for more input/ideas.

[koebi]

Code-wise, this looks good.
Changelog entry is missing.

Could this be added to the routes from the processing algorithms as well? There, we should however switch to random colors, as it currently happens - otherwise routes won't be very discernable…

[TheGreatRefrigerator]

We should check the behaviour with default styles as well. (The default styles set by user).
Not sure if we want to override explicit settings a user made. Or give an additional option to not style the output?

[koebi]

We should check the behaviour with default styles as well. (The default styles set by user). Not sure if we want to override explicit settings a user made. Or give an additional option to not style the output?

We discussed this and settled on overwriting any default style for the "frontend" - people setting a default style know enough to apply their changes after the fact, and processing algorithm behaviour won't be changed.