Fix for Issue #5507- "[Core][layout issue]: The layout of bottom panel and selected file seem strange when drag the left sidebar."

Fix for [Issue #5507](https://github.com/adobe/brackets/issues/5507)- "[Core][layout issue]: The layout of bottom panel and selected file seem strange when drag the left sidebar."

Pass the minimum main window width down to the shell, so that we can enforce a minimum width when resizing the window.