Compatibility with IntelliJ 252 (DefaultActionGroup#getChildren)

[adangel]

The method DefaultActionGroup#getChildren has been deprecated with 242 (JetBrains/intellij-community@76e9f2f).
And has been removed with 252 (JetBrains/intellij-community@e183967).

This should fix the build problems like

Plugin PMDPlugin:2.0.6 against IC-252.13776.59: 2 compatibility problems
Compatibility problems (2): 
    #Invocation of unresolved method com.intellij.plugins.bodhi.pmd.actions.PMDCustom.getChildren(AnActionEvent, ActionManager) : AnAction[]
        Method com.intellij.plugins.bodhi.pmd.PMDProjectComponent.updateCustomRulesMenu() : void contains an *invokevirtual* instruction referencing an unresolved method com.intellij.plugins.bodhi.pmd.actions.PMDCustom.getChildren(com.intellij.openapi.actionSystem.AnActionEvent, com.intellij.openapi.actionSystem.ActionManager) : com.intellij.openapi.actionSystem.AnAction[]. This can lead to **NoSuchMethodError** exception at runtime.
        The method might have been declared in the super classes or in the super interfaces:
          com.intellij.openapi.actionSystem.ActionGroup
          com.intellij.openapi.actionSystem.AnAction
          com.intellij.openapi.actionSystem.DefaultActionGroup
          com.intellij.openapi.actionSystem.ActionUpdateThreadAware
          com.intellij.openapi.project.PossiblyDumbAware
    #Invocation of unresolved method com.intellij.openapi.actionSystem.DefaultActionGroup.getChildren(AnActionEvent, ActionManager) : AnAction[]
        Method com.intellij.plugins.bodhi.pmd.PMDProjectComponent.registerActions(java.lang.String actionName) : com.intellij.openapi.actionSystem.ActionGroup contains an *invokevirtual* instruction referencing an unresolved method com.intellij.openapi.actionSystem.DefaultActionGroup.getChildren(com.intellij.openapi.actionSystem.AnActionEvent, com.intellij.openapi.actionSystem.ActionManager) : com.intellij.openapi.actionSystem.AnAction[]. This can lead to **NoSuchMethodError** exception at runtime.
        The method might have been declared in the super classes or in the super interfaces:
          com.intellij.openapi.actionSystem.ActionGroup
          com.intellij.openapi.actionSystem.AnAction
          com.intellij.openapi.actionSystem.ActionUpdateThreadAware
          com.intellij.openapi.project.PossiblyDumbAware

[adangel]

Note, this is the reason why currently master branch and all pull requests are failing...