Fix broken interactions involving manual time entry and Come/Leave buttons

[markus-walther]

Currently, when using between manual time entry and Come/Leave buttons, anomalies occur.

For example, a filled-in time-interval start does not result in the Leave (Gehen) button being activated:

Also, after a Come/Leave-triggered time pair the Come (Kommen) button looks active, but does not respond to clicks (expected: by opening another time-pair line):

Third, when adding another time-pair line using the plus button, an empty line is correctly added, but the focus is on the top-of-page time-interval start field. In the same situation, the Come button correctly looks active, but actually clicking it can give varying results, like another time-pair line being appended (expected: reuse the last, empty line) or filling in the time-interval end field and remaining active (expected: Come button only ever fills the start field of a line, Leave button active):

Define rigid interaction rules that match user expectations and at least rule out the above anomalies.

Then implement them to fix this issue.

[markus-walther]

Interaction rules:

1. Come button active iff at least one unfilled time-pair start field exists or no incomplete time-pair line exists.
2. Leave button active otherwise.
3. Come button forces appending of a new time-pair line iff 0 time-pair lines or only complete time-pair lines exist.