LogbookWidget countryFilter name filter

[aa5sh]

Added changes to make the countryFilter combobox filterable. #719

[foldynl]

@aa5sh : many thanks. It looks good. I'm thinking a bit about whether to implement it more as a QLineEdit with a Completer. I'm not sure if users like editable combo boxes. I used an editable combo box for DXC Server, and it seems most users still use the default setting.

But this change is definitely a step in the right direction. I need to test it to see whether it searches correctly both with and without diacritics (in languages that use).

[foldynl]

I finally implemented a new widget. I hope it will be clearer and easier to work with than an editable combo or QLineedit completer. I hope the public source code will be available soon.