Changed the logic of First Good Bin box.

Now, instead of synchronizing boxes only when focus is lost,
all of the text changes are synchronized, which means that the start time
will always be up-to-date.

Additionally, I've added validator for the First Good Bin box.

Refs #6990
Change the way start time box is validated.

Now we do it only when it is used. With validator in-place that should be
fine. We are explicitly working with First Good Bin box if appropriate
option is selected, and if it is wrong we reset it to default value.

Refs #6990
Merge remote-tracking branch 'origin/feature/6990_muon_first_bin_autoupdate'