Add 'window' widget type

[airstruck]

This widget can be used as the root widget of a layout to manage properties of the window.

• [ ] Attribute maximized -> SDL_MaximizeWindow / SDL_RestoreWindow
• [ ] Attribute minimized -> SDL_MinimizeWindow / SDL_RestoreWindow
• [ ] Attribute borderless -> SDL_SetWindowBordered
• [ ] Attribute fullscreen -> SDL_SetWindowFullscreen
• [ ] Attribute grab -> SDL_SetWindowGrab (will it work with Love?)
• [ ] Attribute icon (override) -> SDL_SetWindowIcon
• [ ] Attribute maxwidth/maxheight -> SDL_SetWindowMaximumSize
• [ ] Attribute minwidth/minheight (override) -> SDL_SetWindowMinimumSize
• [ ] Attribute top/left (override) -> SDL_SetWindowPosition
• [ ] Attribute width/height (override) -> SDL_SetWindowSize
• [ ] Attribute title -> SDL_SetWindowTitle

[airstruck]

SDL backend should be able to use this to support multiple windows somehow. Calling layout:show on a "window layout" would show it in its own window, but how will "windowless layouts" know which window to show up in?

[airstruck]

Setting minimum/maximum size doesn't seem to do anything under gnome 3, gnome classic or openbox. Not sure what's going on there, or if it works anywhere else. The rest seems to work fine under SDL, still need to add support in Love backend.