Remove ggplot themeing as far as possible

[felix-hof]

For the planned submission to CRAN, we should remove the hardcoded themeing for ggplot geoms as far as possible. This should be customisable by the user.

Ideas include:

• styling arguments for lines (dashed, dotted, etc) for geoms that need to be in the plot (include as arguments to autoplot()?)
• for the rest (i.e. themeing) stick to defaults and let the user add themes by using something like autoplot() + theme() or autoplot() & theme() if length(type) == 1 or length(type) > 1 respectively