Add data search path support

Also add LMMS_DATA_DIR env var to "data:" search paths.
When lmms is launched from its build directory (without `make install`ing),
LMMS_DATA_DIR can be passed to point lmms to the "data" directory in the
source tree.

src/core/BandLimitedWave.cpp

@@ -67,7 +67,7 @@ void BandLimitedWave::generateWaves()
 	int i;
 
 // set wavetable directory
-	s_wavetableDir = ConfigManager::inst()->dataDir() + "wavetables/";
+	s_wavetableDir = "data:wavetables/";
 
 // set wavetable files
 	QFile saw_file( s_wavetableDir + "saw.bin" );

src/core/ConfigManager.cpp

@@ -63,6 +63,9 @@ ConfigManager::ConfigManager() :
 	m_flDir( QDir::home().absolutePath() ),
 	m_recoveryFile( QDir(m_workingDir).absoluteFilePath("recover.mmp") )
 {
+	if (! qgetenv("LMMS_DATA_DIR").isEmpty())
+		QDir::addSearchPath("data", QString::fromLocal8Bit(qgetenv("LMMS_DATA_DIR")));
+	QDir::addSearchPath("data", m_dataDir);
 }