RFR: 8346281: [Windows] RenderScale doesn't update to HiDPI changes [v2]

[Jose Pereda]

> This PR adds the missing native implementation for Windows, `GlassWindow::HandleDPIEvent`, to notify the (Java) window when there is a DPI change event, which can happen when the user changes the resolution of the screen (via Settings -> System -> Display -> scale), while the JavaFX application is running, in a similar way as Java Desktop [handles](https://github.com/openjdk/jdk/blob/master/src/java.desktop/windows/native/libawt/windows/awt_Window.cpp#L1418) this DPI event.
> 
> When such `WM_DPICHANGED` event happens, `GlassWindow::HandleDPIEvent` notifies the (Java) window, which changes its platform scale via `Window::notifyScaleChanged`, and `GlassScreen::HandleDisplayChange();` is needed too, to update the platform scale of the screen where the window is at as well.
> 
> The change in `Screen:: notifySettingsChanged` is needed in order to prevent disposing screens that are not referenced by windows, and only do it when the screen instance of a given window does change.
> 
> There are no tests added to this PR, since these would require manual intervention to change the resolution of the display. In any case, the test case added to the [issue](https://bugs.openjdk.org/browse/JDK-8346281) runs fine now when the app runs on a given screen and the user changes its resolution. It has also been tested that it doesn't have any side effect on macOS.
> 
> For the case when the app runs on a different display that the one that was changed, see https://bugs.openjdk.org/browse/JDK-8371302 and a possible fix with https://github.com/openjdk/jfx/pull/1963.

Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:

  Use set and remove unnecessary casting

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1964/files
  - new: https://git.openjdk.org/jfx/pull/1964/files/9ec63bb5..fbf6c6a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1964&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1964&range=00-01

  Stats: 8 lines in 2 files changed: 2 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1964.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1964/head:pull/1964

PR: https://git.openjdk.org/jfx/pull/1964