Create geoms for lotri/utri_heatmap, *_heatpoint and *_heatcircle

[r-link]

At the moment, these functions are all implemented as stats - this means that the position computed in the compute_panel() step is not validated in the draw_panel() step and may be off if the plotting range is modified by quantities computed from other stats (like CIs for geom_smooth()) or by additional layers with external data.

TODO:

• write geoms with a draw_panel() step analogous to geom_reltext() to replace or complete the existing stats,
• check if stat_corrtext/funtext() can be used to calculate the necessary data - if not, adjust existing stats,
• rewrite the corresponding utri_/lotri_ functions.

[r-link]

Update:

• *_heatmap() and *_heatpoint() have an improved functionality,
• *_heatcircle() still needs a geom.