IDEA-153474 let JDK detect Xft.dpi value on non-GTK Linux DEs

Use the GTK method:

https://developer.gnome.org/gobject/stable/gobject-The-Base-Object-Type.html#g-object-get

to retrieve "gtk-xft-dpi" integer property of the X settings.

Add the property to JDK's GtkEngine & gtk2-interface.
Then read the property via GtkEngine from GTK LaF when "gnome.Xft/dpi" is undefined. It's assumed GTK LaF is forcedly installed.

(cherry picked from commit e05fc391ae0a3cc389e836441f882c0cf6ab3b99)
(cherry picked from commit fd615a5b45)

src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/GTKEngine.java

@@ -94,6 +94,7 @@ class GTKEngine {
     static enum Settings {
         GTK_FONT_NAME,
         GTK_ICON_SIZES,
+        GTK_XFT_DPI,
         GTK_CURSOR_BLINK,
         GTK_CURSOR_BLINK_TIME
     }

src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/PangoFonts.java

@@ -163,6 +163,10 @@ class PangoFonts {
         int dpi = 96;
         Object value =
             Toolkit.getDefaultToolkit().getDesktopProperty("gnome.Xft/DPI");
+
+        if (!(value instanceof Integer)) {
+            value = GTKEngine.INSTANCE.getSetting(GTKEngine.Settings.GTK_XFT_DPI);
+        }
         if (value instanceof Integer) {
             dpi = ((Integer)value).intValue() / 1024;
             if (dpi == -1) {

src/java.desktop/unix/native/libawt_xawt/awt/gtk2_interface.c

@@ -2479,6 +2479,8 @@ static jobject gtk2_get_setting(JNIEnv *env, Setting property)
             return get_string_property(env, settings, "gtk-font-name");
         case GTK_ICON_SIZES:
             return get_string_property(env, settings, "gtk-icon-sizes");
+        case GTK_XFT_DPI:
+            return get_integer_property(env, settings, "gtk-xft-dpi");
         case GTK_CURSOR_BLINK:
             return get_boolean_property(env, settings, "gtk-cursor-blink");
         case GTK_CURSOR_BLINK_TIME:

src/java.desktop/unix/native/libawt_xawt/awt/gtk_interface.h

@@ -316,6 +316,7 @@ typedef enum
 {
     GTK_FONT_NAME,
     GTK_ICON_SIZES,
+    GTK_XFT_DPI,
     GTK_CURSOR_BLINK,
     GTK_CURSOR_BLINK_TIME
 } Setting;