Added resizable window class

[christophe-f8]

I've added a ResizableWindow class that is based on the standard Window class.

The window can be resized horizontally and vertically, but only a vertical scrollbar is implemented so there is a different behavior for width and height:

• Width can not be resized smaller than the minimum width needed to fully display all widgets
• If the width is more than the minimum needed, the layout of the window stretch horizontally
• Height can be resized smaller than the minimum height needed to fully display all widgets if enforceMinimumHeight is set to true in the constructor. In such case a scrollbar appears.
• If the height is more than the minimum needed, a blank area appears at the bottom of the window.

The main thinking behind this behavior is that horizontal scrollbars are probably bad UI design anyway. This might not suit the needs of everyone, but I'm creating this PR should people need this (or want to extend it further)

Hope it helps !

[NeuralDip]

Hello, to me this is a very useful feature.
I have merged it locally to the master branch and it does not have any serious conflicts.
Is this going to be merged? Is there any reason why you don't plan to merge it?
Best Regards

[christophe-f8]

Closing this PR because it now has conflicts and @NeuralDip created a new/better PR #55 .