Warn about LADSPA problems (#7267)

Introduce a method which checks if a file that's loaded has the LADSPA
controls saved in an old format that was written with a version before
commit e99efd541a.

The method is run at the end so that problems in all file versions are
detected. If a real upgrade was to be implemented it would have to run
between `DataFile::upgrade_0_4_0_rc2` and `DataFile::upgrade_1_0_99`.

See #5738 for more details.

If a problematic file is encountered a warning dialog that provides the number of affected LADSPA plugins is shown.

include/DataFile.h

@@ -133,6 +133,7 @@ private:
 	void upgrade_noteTypes();
 	void upgrade_fixCMTDelays();
 	void upgrade_fixBassLoopsTypo();
+	void findProblematicLadspaPlugins();
 
 	// List of all upgrade methods
 	static const std::vector<UpgradeMethod> UPGRADE_METHODS;