Don't reset InputDevicesModel when devices change

Summary:
Instead of reset a proper beginInsertRows, beginRemoveRows is used.
In addition changes in the device are also listened on and emit the
proper dateChanged signal.

Test Plan:
Opened debug console, selected the touchpad device,
toggled shortcut and verified that this updated the data column.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1547