Add MP4::File::strip() to remove meta atom from file

This changes the modifications from the last commit in order to
achieve the following behavior: MP4::File::save() works in the
same way as before, i.e. it will never shrink the file and will
make space from removed items available as padding in the form of
a "free" atom. To completely remove the "meta" atom from the file,
a new method strip() is introduced, which can be used in the same
way as its MPEG::File::strip() counterpart.

tests/test_mp4.cpp

@@ -673,10 +673,8 @@ public:
       MP4::File f(copy.fileName().c_str());
       CPPUNIT_ASSERT(f.isValid());
       CPPUNIT_ASSERT(f.hasMP4Tag());
-      MP4::Tag *tag = f.tag();
-      CPPUNIT_ASSERT(!tag->isEmpty());
-      tag->setTitle("");
-      f.save();
+      CPPUNIT_ASSERT(!f.tag()->isEmpty());
+      f.strip();
     }
     {
       MP4::File f(copy.fileName().c_str());