Improve avoid_negative_ts

From https://github.com/mifi/lossless-cut/discussions/2031#discussioncomment-9834146

• Change that setting automatically, e.g. when Merge is selected, default to make_non_zero; when the mode is changed again, revert to the previous value, be it auto or whatever the user selected. This would save a ton of clicking.
• Unify the tooltips for the various avoid_negative_ts values: if I want to read what each value does, without leaving LosslessCut, I have to cycle manually through each option.
• When make_non_zero is selected, clicking on the warning sign does not open any tooltip, making it impossible to find out what it does from within the program.
• Make it explicit that avoid_negative_ts is not a LosslessCut option (in hindsight it is obvious but as someone who never uses ffmpeg from the command line, I just didn't connect the dots)