Unification of options for all plot functions

[JuleAdrian]

It would be nice, when the user would have similar options in all plot functions.
E.g. whenever a line is plotted, the user should be able to adjust the color, width and opacity of the line. Whenever dots are plotted also the size and maybe the kind of marker should be adjustable.

Linked with task #449

[mirakuepper]

E.g. in plot_time_distance following things would be nice:

• change size of marker of starting positions (would also give the possibility to hide those completely)
• change alpha of "lines" of the time-distance relation
• change "line widths" of the time-distance relation

[mirakuepper]

Open for discussion:
Currently some plot-functions have "title" as optional argument and nonetheless create a fixed title if no own title argument is given. E.g. plot_time_distance keeps writing "Distance Plot" as title... Having the option to set a title is very nice, but it would (in my opinion) be nice if there were no forced titles anyway...