[MSPAINT] Base dialog implementations on CDialogImpl

- Wrap implementation of Mirror/Rotate, Stretch/Skew and Attributes dialog in subclasses of CDialogImpl
- Turn global variables from code that was moved into member functions into member variables
- Add global instances of all three dialog classes
- Use <global instance>.DoModal(...) instead of one-line wrappers
- Replace some TCHAR arrays with CStrings

base/applications/mspaint/main.cpp

@@ -12,11 +12,6 @@
 
 /* FUNCTIONS ********************************************************/
 
-int widthSetInDlg;
-int heightSetInDlg;
-
-STRETCHSKEW stretchSkew;
-
 POINT start;
 POINT last;