Polish IPv4 settings dialog

Summary:
Set the hostname as the placeholder text, which will be used if the hostname is empty.
Show an specialValueText and a suffix on the timeout field.
Make sure we don't try to save the dialog if the parent window has been deleted.

Test Plan: Used it

Reviewers: #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22394