Remove “using namespace std” directive

The file thirdparty\intervaltree\IntervalTree.h contains a using namespace std directive.

Since this file is included in a lot of places, it ends up polluting the global namespace of a large amount of code across both the libmscore and the mscoreapp projects. This namespace pollution has resulted in the recurring problem of std:: prefixes being missing in pull requests. Many of these omissions have slipped through the code review process and are now in committed code. As of this writing there are several dozen instances of this in the codebase.

The using namespace std directive should be removed, and the missing std:: prefixes should be added wherever needed.