Implement tenuto signs

[reinierdevalk]

Examples of star-like signs:

gerle-1552_1-no_4.png

rotta-1546_15-no_23.png

Examples of line-like signs:

vallet-1615-een_en_twintich-no_1.jpg

vallet-1615-secretum_musarum-no_1.jpg

[kepper]

the first two examples can rely on existing <artic>, the last two require something similar to <pedal>. Can we generalize this into a more generic concept?