Migrate LineMarker target to leaf elements to fix performance warning / error

[Haehnchen]

For PhpStorm 2017.3.x LineMarker must be attached to leaf elements see description https://github.com/JetBrains/intellij-community/blob/master/platform/lang-api/src/com/intellij/codeInsight/daemon/LineMarkerProvider.java#L31 for the why.

[junit] ERROR: Performance warning: LineMarker is supposed to be registered for leaf elements only, but got: Class method (class com.jetbrains.php.lang.psi.elements.impl.MethodImpl) instead. First child: Modifier list (class com.jetbrains.php.lang.psi.elements.impl.PhpModifierListImpl)