Make indicator-application hack work in flatpak

Flatpak doesn't share $XDG_RUNTIME_DIR with the host making
QSystemTrayIcon not to work on Xubuntu/Ubuntu MATE/Ubuntu Budgie.

Although, it shares a subfolder, according to
https://docs.flatpak.org/en/latest/sandbox-permissions.html

Amends 0baa26638d

Pick-to: 6.3 6.2 5.15
Change-Id: I2d0043fc5a4c2c51e8fa1a920f3cada3b07eba6d
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Aleix Pol Gonzalez <aleixpol@kde.org>

src/gui/platform/unix/dbustray/qdbustrayicon.cpp

@@ -46,6 +46,7 @@
 #include <QRect>
 #include <QLoggingCategory>
 #include <QStandardPaths>
+#include <QFileInfo>
 #include <QDir>
 #include <QMetaObject>
 #include <QMetaEnum>
@@ -76,8 +77,12 @@ Q_LOGGING_CATEGORY(qLcTray, "qt.qpa.tray")
 static QString iconTempPath()
 {
     QString tempPath = QStandardPaths::writableLocation(QStandardPaths::RuntimeLocation);
-    if (!tempPath.isEmpty())
+    if (!tempPath.isEmpty()) {
+        QString flatpakId = qEnvironmentVariable("FLATPAK_ID");
+        if (!flatpakId.isEmpty() && QFileInfo::exists(QLatin1String("/.flatpak-info")))
+            tempPath += QLatin1String("/app/") + flatpakId;
         return tempPath;
+    }
 
     tempPath = QStandardPaths::writableLocation(QStandardPaths::GenericCacheLocation);