FileUtil: Add a class for Exists/IsDirectory/GetSize

Some code was calling more than one of these functions in a row
(in particular, FileUtil.cpp itself did it a lot...), which is
a waste since it's possible to call stat a single time and then
read all three values from the stat struct. This commit adds a
File::FileInfo class that calls stat once on construction and
then lets Exists/IsDirectory/GetSize be executed very quickly.

The performance improvement mostly matters for functions that
can be handling a lot of files, such as File::ScanDirectoryTree.

I've also done some cleanup in code that uses these functions.
For instance, some code had checks like !Exists() || !IsDirectory(),
which is functionally equivalent to !IsDirectory(), and some
code was using File::GetSize even though there was an IOFile
object that the code could call GetSize on.

Source/Core/DolphinWX/Input/InputConfigDiag.cpp

@@ -856,11 +856,9 @@ void InputConfigDialog::LoadProfile(wxCommandEvent&)
   std::string fname;
   InputConfigDialog::GetProfilePath(fname);
 
-  if (!File::Exists(fname))
-    return;
-
   IniFile inifile;
-  inifile.Load(fname);
+  if (!inifile.Load(fname))
+    return;
 
   controller->LoadConfig(inifile.GetOrCreateSection("Profile"));
   controller->UpdateReferences(g_controller_interface);