Use value of env ESDE_APPDATA_DIR if present (#74)

docs/FRONTENDS.md

@@ -81,6 +81,12 @@ media files into the `downloaded_media` folder (e.g.
 `~/ES-DE/downloaded_media/<PLATFORM>/screenshots/` for screenshots) from where
 ES-DE will pick them up. Note that ES-DE does not support textures currently.
 
+!!! tip
+
+    You may override the default storage folder of ES-DE (`/home/<USER>/ES-DE`)
+    on a non-Windows build by setting the environment variable `ESDE_APPDATA_DIR`
+    to a different folder as documented in the ES-DE documentation [here](https://gitlab.com/es-de/emulationstation-de/-/blob/v3.0.2/INSTALL.md#changing-the-application-data-directory).
+
 All the gory details of ES-DE's gamelist file format you can find in the section
 "Gamelist Reference"
 [here](https://gitlab.com/es-de/emulationstation-de/-/blob/master/INSTALL.md?ref_type=heads#gamelistxml).

src/esde.cpp

@@ -23,12 +23,21 @@
 #include "gameentry.h"
 
 #include <QDir>
+#include <QProcessEnvironment>
 #include <QStringBuilder>
 #include <QStringList>
 
 Esde::Esde() {}
 
 inline const QString baseFolder() {
+#if !(defined Q_OS_WIN || defined Q_OS_ANDROID)
+    // https://gitlab.com/es-de/emulationstation-de/-/blob/v3.0.2/INSTALL.md#changing-the-application-data-directory
+    QString appdata =
+        QProcessEnvironment::systemEnvironment().value("ESDE_APPDATA_DIR", "");
+    if (!appdata.isEmpty()) {
+        return appdata;
+    }
+#endif
     return QString(QDir::homePath() % "/ES-DE");
 }