Bug 1856627 [Linux] Use BTN_FORWARD (mouse button 10) to web navigate forward r=emilio

Recently we use BTN_SIDE (Button 8) to navigate web back, BTN_EXTRA (Button 9) to navigate web forward.
In this patch add BTN_FORWARD (Button 10) to also navigate web forward.

Differential Revision: https://phabricator.services.mozilla.com/D190153

widget/gtk/nsWindow.cpp

@@ -3459,7 +3459,7 @@ nsresult nsWindow::SetTitle(const nsAString& aTitle) {
   if (!mShell) return NS_OK;
 
     // convert the string into utf8 and set the title.
-#define UTF8_FOLLOWBYTE(ch) (((ch)&0xC0) == 0x80)
+#define UTF8_FOLLOWBYTE(ch) (((ch) & 0xC0) == 0x80)
   NS_ConvertUTF16toUTF8 titleUTF8(aTitle);
   if (titleUTF8.Length() > NS_WINDOW_TITLE_MAX_LENGTH) {
     // Truncate overlong titles (bug 167315). Make sure we chop after a
@@ -4686,7 +4686,7 @@ void nsWindow::OnButtonPressEvent(GdkEventButton* aEvent) {
     case 7:
       NS_WARNING("We're not supporting legacy horizontal scroll event");
       return;
-    // Map buttons 8-9 to back/forward
+    // Map buttons 8-9(10) to back/forward
     case 8:
       if (!Preferences::GetBool("mousebutton.4th.enabled", true)) {
         return;
@@ -4694,6 +4694,7 @@ void nsWindow::OnButtonPressEvent(GdkEventButton* aEvent) {
       DispatchCommandEvent(nsGkAtoms::Back);
       return;
     case 9:
+    case 10:
       if (!Preferences::GetBool("mousebutton.5th.enabled", true)) {
         return;
       }