src: implemented move constructor/assignment for filepath

* improves indexing performance by roughly 5%
* switched to use uniqu_ptr for boost path to get rid of including boost in header
* added different methods to change the current FilePath or to make a copy and change that one
* used these methods appropriately throughout the code base
* removed exists() method from FileSystem because it is already contained in FilePath
* added const in some places

src/lib_gui/qt/window/QtLicenseWindow.cpp

@@ -134,7 +134,7 @@ void QtLicenseWindow::populateWindow(QWidget* widget)
 void QtLicenseWindow::windowReady()
 {
 	m_content->setStyleSheet(m_content->styleSheet() +
-			utility::getStyleSheet(ResourcePaths::getGuiPath().concat(FilePath("license/license.css"))).c_str());
+			utility::getStyleSheet(ResourcePaths::getGuiPath().concatenate(FilePath("license/license.css"))).c_str());
 
 	addLogo();