plugins: settingsadapter: make all properties constant to avoid QML errors

Fixes the following runtime errors:

QQmlExpression: Expression qrc:///qml/main.qml:121 depends on non-NOTIFYable properties:
    luna::SettingsAdapter::displayWidth
    luna::SettingsAdapter::displayHeight
QQmlExpression: Expression qrc:///qml/main.qml:93 depends on non-NOTIFYable properties:
    luna::SettingsAdapter::displayFps

Signed-off-by: Simon Busch <morphis@gravedo.de>
plugins: lunaserviceadapter: implement proper error handling

Signed-off-by: Simon Busch <morphis@gravedo.de>
Merge pull request #32 from webOS-ports/morphis/work

Fixes and proper luna service error handling