[Fix] 20 Autocomplete Bug (alternative solution)

[Apollo3zehn]

During my review of PR #149 ([Fix] 20 Autocomplete Bug) I tried to understand the root cause of the problem and finally found it.

Here is the problem description:

1. When users types into the autocomplete input for the first time, they are able to do so successfully. E. g. typing 10min works.
2. But the second time a value is being entered, it usually fails and the value is replaced with 1 s.

The root cause of this behavior is the following:

1. Initially the autocomplete input shows the value 1 s. When a user types e.g. 10 min there will be internal exceptions thrown for every keystroke until 10 min is entered completely. Whenever an exception is thrown (because partial values like 10 mi are invalid), the return value is new Period(default) which equals to 1 s as shown here:
   

   nexus/src/Nexus.UI/ViewModels/SettingsViewModel.cs

   Line 77 in 98ea2f0

   ? TimeSpan.FromSeconds(1)

2. This means that as long as incomplete (= invalid) values are being entered, the autocomplete input value does not change, internally it always has the value of 1 s.
3. As long as the internal value does not change, the user can continue entering characters until the input value becomes valid
4. However, when, for instance, the user successfully entered 10 min and then tries to enter 5 s, this immediately fails because - as described above - partial values like 5 are invalid and cause exceptions to be thrown. This in turn leads to a return value of new Period(default) which in turn equals to 1 s, so now the internal value does change and the input is immediately replaced with 1 s and the the user cannot continue entering the 5 s value.

So the obvious solution is to not return default or new Period(default) but the current value instead until the new input is valid. The current value is not directly accessible in the converter but a getCurrentValue function can be passed into the converter's constructor to work around this problem as I did in the first commit.

[Apollo3zehn]

I have renamed PeriodConverter.cs to Period.cs which makes it a bit harder to see what has actually changed :-(