Replace QWorkspace with QMdiArea

[wmayer]

In Qt version 4.3 the class QMdiArea was introduced to replace the old-fashioned QWorkspace. Since we have two annoying problems with QWorkspace I started to replace it step by step with QMdiArea in the FreeCAD sources. These two problems are:

• When the HTML start page is shown at startup and a 3d view is created there is always an area which isn't correctly repainted until you manually switch to another page.
  (There is also a crude hack to do that programmatically when adding the 3d view inside MainWindow::addWindow)
• When a 3d view is not maximized it's not possible to interact with the scene because it also move the whole widget

With the new QMdiArea there are no more the Minimize/Maximize/Close buttons. Instead you'll find the Close button directly in the associated tab.

[yorik]

Instead you'll find the Close button directly in the associated tab

Which is even better I think, more firefox-like (or should we say google chrome-like now )