Extend the canvas.Window class to create a simple window decoration
if a type for it (currently every type maps to the same style) is
given. It supports moving the window by dragging inside the title
bar and setting a window title.
- std.string:
* Add method compare
* Add method starts_with
- canvas.Group:
* Add method rect for drawing (rounded) rectangles
- Rename canvas.Dialog to canvas.Window to free the name Dialog
for real dialogs.
