Mark settings and SMS app as single window

There is only ever one window of those

This was shells know that launching a second instance makes no sense and can disable the relevant actions

(cherry picked from commit 6ac5e8b0)

settings/org.kde.kdeconnect-settings.desktop

@@ -94,3 +94,4 @@ GenericName[zh_CN]=连接并同步您的设备
 GenericName[zh_TW]=連線並且同步您的裝置
 Categories=Qt;KDE;Settings;HardwareSettings;
 NotShowIn=KDE;
+X-GNOME-SingleWindow=true

smsapp/org.kde.kdeconnect.sms.desktop

@@ -123,3 +123,4 @@ Categories=Qt;KDE;Network;InstantMessaging
 
 X-DBUS-StartupType=Unique
 X-DBUS-ServiceName=org.kde.kdeconnect.sms
+X-GNOME-SingleWindow=true