Respect XDG_CONFIG_HOME on Linux

The application's configuration is stored using `ApplicationProperties`
which doesn't consider the *XDG Base Directory Specification*. There is
`File::userApplicationDataDirectory` but it also doesn't respect it
according to an upstream [bug report].

Supporting the full specification is more effort but just supporting
`XDG_CONFIG_HOME` already addresses the common issue of the home
directory being spammed with configuration files. So just read that
environment variable or use its default to set the `folderName`, with an
appended application name since the specification suggests to also use
a subfolder.

This is a breaking change for Linux users but since none are known at
the time it seems acceptable to just switch instead of supporting some
form of migration. It's also mentioned in the change log.

[bug report]: juce-framework/JUCE#1184

Author: ServiusHack

Changelog.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+* *Breaking Change*: Respect `XDG_CONFIG_HOME` on Linux.
+
 ## [1.2.0] - 2025-04-15
 
 ### Added

Source/Main.cpp

@@ -31,6 +31,16 @@ class MStarPlayerApplication : public juce::JUCEApplication
         juce::PropertiesFile::Options options;
         options.applicationName = "MStarPlayer";
         options.filenameSuffix = ".settings";
+#ifdef JUCE_LINUX
+        if (const char* path = std::getenv("XDG_CONFIG_HOME"))
+        {
+            options.folderName = path + juce::String("/MStarPlayer");
+        }
+        else if (const char* home = std::getenv("HOME"))
+        {
+            options.folderName = home + juce::String("/.config/MStarPlayer");
+        }
+#endif
         m_applicationProperties.setStorageParameters(options);
 
         if (m_applicationProperties.getUserSettings()->getValue("language") == "de")