mpris: define variables only when required

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
mpris: use QSettings intead of PlainConfigFile

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
mpris: use QSettings intead of PlainConfigFile

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
themes: use QSettings intead of PlainConfigFile

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
configuration: remove unused class PlainConfigFile

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>