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 e99efd5.

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 LMMS#5738 for more details.

…ialLadspaPluginProblems

Conflicts:
* include/DataFile.h
* src/core/DataFile.cpp

Count the number of problematic LADSPA plugins and show it in the
warning.