Refactor and fix the mobile wifi kcm

Summary:
Previous refactoring and existing errors made editing connections impossible. Also, existing work for adding hidden networks was broken. By fixing these problems, this patch implements T12893.

The checks for signal strength being > 0 were removed to make make hidden networks show up in the list

Test Plan: Test adding and editing connections

Reviewers: #plasma, ngraham, jgrulich, jbbgameich

Reviewed By: jgrulich, jbbgameich

Subscribers: nicolasfella, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28799