RFR: 8269429: Linux: Only the last APPLICATION_MODAL window behaves correctly [v2]

[Thiago Milczarek Sayao]

> The PR approach is to set  `gtk_window_set_keep_above` to true on APPLICATION_MODAL windows, so they will not stay behind non APPLICATION_MODAL windows.
> 
> This is passed on WindowStage.java:198 as a mask.
> 
> The weird thing is that `_enterModal()` is never called. This seems the right function to be called for `APPLICATION_MODAL`, as `_enterModalWithWindow` fits for `WINDOW_MODAL`.

Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:

  Reuse existing set_modal (which was previously unused).

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/551/files
  - new: https://git.openjdk.java.net/jfx/pull/551/files/7b483449..612a61ea

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=551&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=551&range=00-01

  Stats: 7 lines in 1 file changed: 3 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/551.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/551/head:pull/551

PR: https://git.openjdk.java.net/jfx/pull/551