Provide Quit handler for system menu bar

[Florian Kirmaier]

Hi Everyone,

In one project, we have to handle the quit-logic for MacOS ourselves,
when the <quit app> menu entry is used.
As background information - this menu entry is set in the
class com.sun.glass.ui.mac.MacApplication.
It's basically hard coded. Currently, in this project, the menu entry
doesn't work in some cases.

My Solution would be:

Provide a method "Platform.setQuiteHandler(Supplier<Boolean>)"
This handler is called when quit <appname> from the menu is called.
If the handler returns true, all windows are closed. Otherwise, nothing
happens.

It would look like the following:
```
/**
 * Sets the handler to be called when the application is about to quit.
 * Currently, this can only happen on MacOS.
 *
 * This handler is called, when the user selects
 * the "Quit <appname>" from the application menu.
 * When the provided handler returns true,
 * the application will close all windows.
 * If the handler returns false, the application will not quit.
 *
 * @param The new quit handler.
 */
public static void setQuitHandler(Supplier x) {
    ...
}
```
I've created a ticket for this topic.
https://bugs.openjdk.org/browse/JDK-8293700

I've got a working version for this change.
According to Kevin Rushforth this need a prior dicussion on the mailing
list.
Any opinion regarding this?

I could provide a pullrequest, if someone is interested.

Florian Kirmaier
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