ProjectExplorer: Shorten space needed to store a FileType

Use class enum to shorten the FileType to quint16. This frees up a couple
of bytes per FileNode and we can have many of those.

Change-Id: I3a9ae25059690fefa15305a4268269647d6dc1c9
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/pythoneditor/pythoneditorplugin.cpp

@@ -598,7 +598,7 @@ class PythonFileNode : public FileNode
 {
 public:
     PythonFileNode(const Utils::FileName &filePath, const QString &nodeDisplayName)
-        : FileNode(filePath, SourceType, false)
+        : FileNode(filePath, FileType::Source, false)
         , m_displayName(nodeDisplayName)
     {}