Removed usage of ASCII C strings in several places

Still QString::toAscii() & friends were used in several places causing
problems such as failed access to files with non-ASCII characters in
name.

Closes #2884115.

plugins/flp_import/FlpImport.cpp

@@ -2027,7 +2027,8 @@ extern "C"
 // neccessary for getting instance out of shared lib
 Plugin * PLUGIN_EXPORT lmms_plugin_main( Model *, void * _data )
 {
-	return new FlpImport( static_cast<const char *>( _data ) );
+	return new FlpImport( QString::fromUtf8(
+									static_cast<const char *>( _data ) ) );
 }
 
 }

plugins/midi_import/MidiImport.cpp

@@ -572,12 +572,10 @@ extern "C"
 // neccessary for getting instance out of shared lib
 Plugin * PLUGIN_EXPORT lmms_plugin_main( Model *, void * _data )
 {
-	return new MidiImport( static_cast<const char *>( _data ) );
+	return new MidiImport( QString::fromUtf8(
+									static_cast<const char *>( _data ) ) );
 }
 
 
 }
 
-
-#undef pos
-#undef setValue

plugins/patman/patman.cpp

@@ -215,7 +215,7 @@ patmanInstrument::LoadErrors patmanInstrument::loadPatch(
 {
 	unloadCurrentPatch();
 
-	FILE * fd = fopen( _filename.toAscii().constData() , "rb" );
+	FILE * fd = fopen( _filename.toUtf8().constData() , "rb" );
 	if( !fd )
 	{
 		perror( "fopen" );