Fix casing of filenames and code in plugins/ (#6350)

No functional changes! No changes to savefiles/presets!

Fixes casing of everything that is currently lowercase but should
be uppercase.

Fixes also some other plugin strings, especially:

* opl2 -> OpulenZ (see 289887f4fc)
* calf -> veal (see ae291e0709)
* ladspa_effect -> LadspaEffect (see 9c9372f0c8)
* remove flp_import (see 2d1813fb64)

doc/Doxyfile.in

@@ -784,9 +784,8 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = plugins/zynaddsubfx/src/ \
-                         plugins/zynaddsubfx/fltk/ \
-                         plugins/ladspa_effect
+EXCLUDE                = plugins/ZynAddSubFx/zynaddsubfx/src/ \
+                         plugins/LadspaEffect
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded