Improve label handling in plots

[HenriekeKotthoff]

Implement angle and truncation in constants?

• get_angle() --> VLKR_PLOT_ANGLE
• trunc_labels() --> VLKR_PLOT_TRUNC

[HenriekeKotthoff]

Example
plot_metrics_items_cor_items(ds, starts_with("empower_"), starts_with("empower"))