Dashboard/Entry time dropdown: keep end time fixed when changing duration

[raph-topo]

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Currently, on Dashboard, the (very sleek) entry start & end time dropdown works as follows:

changing	changes	stays put
duration	end	start
start	duration	end
end	duration	start

Which basically makes the duration cell useless, because if one sets it, one probably overwrites it immediately with either of the two other values

But, since one enters always a task after it has been finished, it would be more logical that end time (i.e. now) stayed put when changing duration (note 1):

changing	changes	stays put	note
duration	start	end	1
start	end	duration	2
end	duration	start	3

Note 2: progression is now left to right & allows for tasks that ran for some time and finished before now to be entered. Note 3: this has to stay to maintain the ability to set both start & end time, calculating duration.

This also applies to Time entries/Add time entry popup.

[raph-topo]

#18 makes sense to be implemented as well regarding this