Linux-specific: Optionally store history in XDG_DATA_HOME in Qt5 build

Separate data directory is only used if history file isn't already
present in the configuration directory.

This commit together with two previous ones allows to switch to
layout described in XDG Base Directory specification. Data is not
migrated automatically and old ~/.goldendict home dir has precedence.
If you wish to use standards-compliant locations, move files and
directories manually:

```
CONFIG="${XDG_CONFIG_HOME:-$HOME/.config}/goldendict"
CACHE="${XDG_CACHE_HOME:-$HOME/.cache}/goldendict"
DATA="${XDG_DATA_HOME:-$HOME/.local/share}/goldendict"
mkdir -p "${CACHE}"
mkdir -p "${DATA}"
mv ~/.goldendict/index "${CACHE}"
mv ~/.goldendict/history "${DATA}"
mv ~/.goldendict/ "${CONFIG}"
```

config.cc

@@ -28,10 +28,31 @@
 #include <QDesktopServices>
 #endif
 
+#if defined( HAVE_X11 ) && QT_VERSION >= QT_VERSION_CHECK( 5, 0, 0 )
+// Whether XDG Base Directory specification might be followed.
+// Only Qt5 builds are supported, as Qt4 doesn't provide all functions needed
+// to get XDG Base Directory compliant locations.
+#define XDG_BASE_DIRECTORY_COMPLIANCE
+#endif
+
 namespace Config {
 
 namespace
 {
+#ifdef XDG_BASE_DIRECTORY_COMPLIANCE
+  const char xdgSubdirName[] = "goldendict";
+
+  QDir getDataDir()
+  {
+    QDir dir = QStandardPaths::writableLocation( QStandardPaths::GenericDataLocation );
+    dir.mkpath( xdgSubdirName );
+    if ( !dir.cd( xdgSubdirName ) )
+      throw exCantUseDataDir();
+
+    return dir;
+  }
+#endif
+
   QString portableHomeDirPath()
   {
     return QCoreApplication::applicationDirPath() + "/portable";
@@ -52,12 +73,12 @@ namespace
       result = QDir::fromNativeSeparators( QString::fromWCharArray( _wgetenv( L"APPDATA" ) ) );
     #else
       char const * pathInHome = ".goldendict";
-      #if QT_VERSION >= QT_VERSION_CHECK( 5, 0, 0 ) && defined( HAVE_X11 )
+      #ifdef XDG_BASE_DIRECTORY_COMPLIANCE
         // check if an old config dir is present, otherwise use standards-compliant location
         if ( !result.exists( pathInHome ) )
         {
           result.setPath( QStandardPaths::writableLocation( QStandardPaths::ConfigLocation ) );
-          pathInHome = "goldendict";
+          pathInHome = xdgSubdirName;
         }
       #endif
     #endif
@@ -2214,7 +2235,7 @@ QString getIndexDir() THROW_SPEC( exError )
 {
   QDir result = getHomeDir();
 
-#if QT_VERSION >= QT_VERSION_CHECK( 5, 0, 0 ) && defined( HAVE_X11 )
+#ifdef XDG_BASE_DIRECTORY_COMPLIANCE
   // store index in XDG_CACHE_HOME in non-portable version
   // *and* when an old index directory in GoldenDict home doesn't exist
   if ( !isPortableVersion() && !result.exists( "index" ) )
@@ -2238,7 +2259,18 @@ QString getPidFileName() THROW_SPEC( exError )
 
 QString getHistoryFileName() THROW_SPEC( exError )
 {
-  return getHomeDir().filePath( "history" );
+  QString homeHistoryPath = getHomeDir().filePath( "history" );
+
+#ifdef XDG_BASE_DIRECTORY_COMPLIANCE
+  // use separate data dir for history, if it is not already stored alongside
+  // configuration in non-portable mode
+  if ( !isPortableVersion() && !QFile::exists( homeHistoryPath ) )
+  {
+    return getDataDir().filePath( "history" );
+  }
+#endif
+
+  return homeHistoryPath;
 }
 
 QString getFavoritiesFileName() THROW_SPEC( exError )

config.hh

@@ -758,6 +758,7 @@ private:
 };
 
 DEF_EX( exError, "Error with the program's configuration", std::exception )
+DEF_EX( exCantUseDataDir, "Can't use XDG_DATA_HOME directory to store GoldenDict data", exError )
 DEF_EX( exCantUseHomeDir, "Can't use home directory to store GoldenDict preferences", exError )
 DEF_EX( exCantUseIndexDir, "Can't use index directory to store GoldenDict index files", exError )
 DEF_EX( exCantReadConfigFile, "Can't read the configuration file", exError )