Adjust NcDatetimePicker styling

[raimund-schluessler]

This PR adjust the NcDatetimePicker styling to bring it inline with the other picker and input styles. It also replaces the icons used with material design icons.

Before	After

Closes #3809.

[susnux]

The border looks darker than the others, but I think this is because of #3814

[raimund-schluessler]

The border looks darker than the others, but I think this is because of #3814

Yes, exactly. I rebased the branch with #3814 in and updated the screenshot. It looks the same now.

[jancborchardt]

@raimund-schluessler just a super detail → when used in the action menu, is the text of the datepicker a bit too far down vertically inside the input? And the "calendar" icon on the right of the input is also not the same position for both of the actions for some reason?

[raimund-schluessler]

when used in the action menu, is the text of the datepicker a bit too far down vertically inside the input?

Might be, but I think this comes from the datepicker styling. See the standalone datepicker:

But to be honest, I find it difficult to say what's the correct position, so I would leave it up to you to fix it 🙈

And the "calendar" icon on the right of the input is also not the same position for both of the actions for some reason?

Yes, it is. And until now, I thought we had no chance to fix this, as it's an icon coming from the browser. But I just noticed it reacts to the padding of the input field. So I will prepare a PR to fix it.

[raimund-schluessler]

@jancborchardt See #3826.