Fix mistakes in FX API docs

This bug will track simple javadoc fixes / typos for openjfx12.

1. Duplicated word in ContextMenu#show(Node,double,double)

    ...adjustments are made to bring the ContextMenu **back back** on screen

2. Animation#status: "In Animation" -> "An Animation"

3. TableColumnHeader#getClassCssMetaData "Returnst" -> "Returns"

4. GridPane#add (both overloads): "column,row" missing space

5. GridPane class docs, under "Row/Column Sizing": "child.However" missing space

6. Duration class docs: "Duration's" -> "Durations" (or better phrasing)

7. Application#launch (both overloads): "main method()". Will fix the formatting on the way (like "Platform.exit")

8. TableView#get/setColumnResizePolicy: "This is the function called"

9. Region#setPadding(Insets value): "By default padding is Insets.EMPTY"

10. ScrollPane: "If an application wants ..., they need to" -> "it needs to"

11. Deferred to JDK-8217270

12. JDK-8216507 RETRACTED

13. Some control classes like ProgressBar contains the line "Implementation of <ControlName> According to JavaFX UI Control API Specification" at the end. It should be removed.

14. ChoiceBox class doc: "It means configuration of the ChoiceBox is order independent." -> It means that the configuration..."
      Missing period after "ChoiceBox item selection is handled by SelectionModel".

15. Transition and Timeline have too many empty spaces around the code snippets.