[API Break] Fix pathTools (and a bit stringTools).

Api changes :
 - removeLastPathElement do not takes extra arguments
   `removePreSeparator` and `removePostSeparator`.
   This is not needed as path do not need special tailing separator.
 - Only one function `split`. Arguments can be implicitly convert to
   string. No need for overloading functions to explicitly cast them.
 - `split` function takes another argument `trimEmpty`. If true, empty
   element are removed.

Path manipulation now almost pass trough a vector<string> to store each
path's part.

Most of the complex works is now made in the normalizeParts function.

src/library.cpp

@@ -115,7 +115,7 @@ unsigned int Library::getBookCount(const bool localBooks,
 
 bool Library::writeToFile(const std::string& path)
 {
-  auto baseDir = removeLastPathElement(path, true, false);
+  auto baseDir = removeLastPathElement(path);
   LibXMLDumper dumper(this);
   dumper.setBaseDir(baseDir);
   return writeTextFile(path, dumper.dumpLibXMLContent(getBooksIds()));