Allow user to change default threshold in percent

[Carolyn Eng]

In detection settings, the default threshold in percent is 8 but I find for most of my tracking, a higher value is optimal (I generally use 16). Is there a way I can change the default setting? If not, can a feature be added?

[Benjamin Knorlein]

fixes #162 : A default threshold for new markers can be set in the Settings dialog

→ <<cset 7e914658cf07>>