Use NcSelect for NcTimezonePicker

[raimund-schluessler]

This PR moves NcTimezonePicker from using NcMultiselect to NcSelect. Since NcSelect does not support grouping, I had to work around this a bit, but the outcome is the same as when using NcMultiselect. I additionally enhanced the search function a bit, so one can now also search for the continent name and see all timezones e.g. in Europe, and searching for "Europe London", will give the correct timezone.

The styling probably needs a bit of adjustment still, because it currently uses the default NcSelect style with a gray/blue border, whereas the current timezone picker has no border. But maybe we want to update the style here and match with the other pickers, so I left it for now.

Before	After

Fixes 1/3 of #3743.

[jancborchardt]

Very cool! :) I think the blue border around it is actually nice.

3 things:

• Style-wise it coukd look a bit more like the ActionMenu component, with the entries not expanding the whole width, but having a bit of margin and having a birder-radius
• The contrast of the highlight seems to not be enough for accessibility, here we should also do it however we make the ActionMenu entry hover/focus accessible cc @JuliaKirschenheuter @Pytal
• Detail: The height of the co tainer should (if possible) always cut the list at ½ of the last entry, so that one entry is cut off and it is obvious to people that the list continues. :)

[raimund-schluessler]

• Style-wise it coukd look a bit more like the ActionMenu component, with the entries not expanding the whole width, but having a bit of margin and having a birder-radius

• The contrast of the highlight seems to not be enough for accessibility, here we should also do it however we make the ActionMenu entry hover/focus accessible cc @JuliaKirschenheuter @Pytal

• Detail: The height of the co tainer should (if possible) always cut the list at ½ of the last entry, so that one entry is cut off and it is obvious to people that the list continues. :)

I think this is all valid feedback, but it affects the NcSelect component where theses styles are coming from. See here a plain NcSelect component, which shares the same inconveniences:

So I would rather not touch this here and make it a separate issue/PR, see #3798.

[jancborchardt]

Sounds good, thanks for opening the issue, approved design-wise then. :)

[raimund-schluessler]

I adjusted the search code a bit, so that the continent labels are shown if one region matches, just like with the old picker.