[Touchpad KCM] Load previous setting on reboot

Summary:
Earlier Previous setting were not loaded on reboot instead default settings were loaded.
So, added config file for saving the settings.

Now on startup, settings from config file will be loaded.

Test Plan: Tested using kcminit and manually rebooting.

Reviewers: ngraham, romangg, davidedmundson

Subscribers: fvogt, davidedmundson, nicolasfella, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21228