POM: readd Trainable_Segmentation

There is now a valid release version with reproducible build!

See:
https://github.com/fiji/Trainable_Segmentation/commit/930ba997b962963cd9b98b14118071fdcc0d14e9
https://github.com/fiji/Trainable_Segmentation/commit/748517d171d1445077027ba64f5d45addebc670d
https://github.com/fiji/Trainable_Segmentation/commit/7cfcdb1912f391036bc0b3d8b8302164ca329025