[MSPAINT][ATL] Encapsulation: mainWindow (#5178)

- Add DoCreate methods to CFullscreenWindow, CMiniatureWindow, and CMainWindow classes.
- Do encapsulation around mainWindow and _tWinMain.
- Add GetOpenFileName, GetSaveFileName, and ChooseColor helper methods to CMainWindow class.
- Move some code in WinMain into CMainWindow::OnCreate.
- Delay creation of CFullscreenWindow and CMiniatureWindow.
- Extend ATL CImage class as CImageDx in newly-created atlimagedx.h of mspaint.
CORE-18867

base/applications/mspaint/dib.h

@@ -31,3 +31,5 @@ HBITMAP SetBitmapAndInfo(HBITMAP hBitmap, LPCTSTR name, DWORD dwFileSize, BOOL i
 HBITMAP Rotate90DegreeBlt(HDC hDC1, INT cx, INT cy, BOOL bRight);
 
 HBITMAP SkewDIB(HDC hDC1, HBITMAP hbm, INT nDegree, BOOL bVertical);
+
+float PpmFromDpi(float dpi);