RFR: 8359396: [Linux] Require Gtk3 >= 3.20 for glass-gtk [v2]

[Thiago Milczarek Sayao]

> Upgrade the minimum required GTK version for JavaFX to GTK 3.20 to enable modern features and better Linux desktop integration.
> 
> JavaFX currently depends on GTK 3.8, [released](https://mail.gnome.org/archives/gtk-devel-list/2013-March/msg00108.html) in March 2013. This version is outdated and predates many useful GTK API improvements.
> 
> GTK 3.20 was [released](https://mail.gnome.org/archives/gtk-list/2016-March/msg00026.html) on March 21, 2016.
> 
> Updating the GTK minimum requirement to 3.20 would enable JavaFX to support new features, including the improvements proposed in  #1605 
> 
> Major Linux distributions already provide GTK 3.20 or newer:
> - Ubuntu LTS 18.04+ (ships GTK 3.22+)
> - Debian 9+ (ships GTK 3.22+)
> - Fedora 24+ (ships GTK 3.20+)
> - Oracle Linux and Red Hat Enterprise Linux 8+ (ships GTK 3.22+)

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

  Add Comment about gtk 3

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1825/files
  - new: https://git.openjdk.org/jfx/pull/1825/files/e1dc7e47..ab611660

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1825&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1825&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1825.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1825/head:pull/1825

PR: https://git.openjdk.org/jfx/pull/1825