Improve password dialogs

Summary:
Improve UI of all paswords dialogs. Previously we had two methods to setup UI
of the dialog, either for VPN or for regular connections like WiFi. I merged
them into one and both are now based on same UI, except that VPN connections
insert their specific widget into the dialog, while regular connections use
only one password field.

Screenshots of old password dialogs:

Screenshots of new password dialogs:

Reviewers: VDG, Plasma, mart

Reviewed By: mart

Subscribers: lukas, fabianr, mart, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5382