improve input for seeding relation

[krichter]

Currently the seeding relation is controlled with a spinner that

• allow specifying values -0.9, -0,8, ..., -0.1 which don't make sense (does 0.0 make sense?)
• doesn't explain that unlimited seeding corresponds to -1

Either a label explaining -1 should be added and the steps -0.9, -0.8, ..., -0.1 should be remoted or the spinner should be limited to the interval [0;infinite] or [0.1;infinite] and a checkbox for "unlimited" (deactivating the spinner) should be added (the latter is more intuitive, but requires more testing).

[Cas]

This is simply an issue with how the custom extjs spinners work and not an intended outcome. It might be rather complicated to implement.

Also see related ticket #2070

[Cas]

Milestone renamed