KCM Notifications : Manage app-specific notifications with KCconfigXT's magic

Summary:
App-specific notifications' behavior are now managed with KConfigXT. The default values are set in plasmanotifyrc shipped with the libnotificationmanager and can be override.
Immutability can be set at applicatioin level in plasmanotifyrc.
Reinitialize button reset all modifications not applied. Default button reset to default applications' behavior.

Require D27059 D27133 D27155

Test Plan:
To set immutable an application or a service specific notifications, add [$i] after an element in `plasmanotifyrc`

* Set immutability for all services (same for applications)
```
[Services][$i]
```
* Set immutability for an app (same for a service)
```
[Applications][org.kde.kdevelop][$i]
```
* Set immutability for a specific entry
```
[Applications][org.kde.kdevelop]
ShowBadges=false
ShowPopupsInDndMode[$i]=true
```

Reviewers: #plasma, ervin, broulik, bport, meven

Reviewed By: #plasma, ervin, broulik, bport

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27188