Add a minimize button to UIWindow

[void4]

Is your feature request related to a problem? Please describe.
Sometimes, a window is opened by a button, and should be hidden by clicking the same button.

Describe the solution you'd like
A function that makes the window and all components within it temporarily invisible and inactive.

Describe alternatives you've considered
It is possible to just move the window position out of the visible screen, it works, but that's not elegant.

[MyreMylar]

I think I could probably add something like this.

It'd mostly just be wrapping pygame.sprite.kill() and then an 'unhide()' function to re-add it to whatever drawing group it's part of. I'll bundle it up with #95 when I get to that.

[void4]

It would great if one could also optionally disable/remove the "close" button on the window frame and replace it with/append a "minimize" button that would have the hiding functionality described above.

[ylenard]

Show and hide methods for UIWindow were implemented in Pull Request #136

[MyreMylar]

Think I might rename this to be about adding a 'minimize' button now that show/hide is in the master branch.

[MyreMylar]

I guess the main difficulty with a minimize button is that without making a whole OS structure for windows there is no obvious way to unhide windows hidden in this way. I guess I could make a new element that works a bit like a taskbar for windows and you could include or not include it at your peril when using windows - or perhaps I should leave that up to users to figure out?

[ylenard]

I think minimize button won't make any sense without a task bar element.

Perhaps it should be possible to choose what happens when the "close" button is clicked - this way the user would be able to make use of the UIWindows and create them only once without altering existing UI structure.

I do believe that TaskBar element would be a useful addition to the module though, but I don't think it's the most important widget that's needed currently - I'd say Tabs > Spinner > TaskBar.