Prevent the AddOns's Dialogs from being inaccessible

[Lisias]

As I suggested on Forum.

if the user change the screen resolution, or accidentally move the Window to outside the visible area, this screen would be unaccessible…

Check the Window's boundaries and be sure the whole Window is visible when the user show the dialog.

hint: check if the Window is wider or taller than the Screen, and add Scrollbars.

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Transferred from TweakScale/TweakScale#239

[Lisias]

I removed the milestone because, currently, all KSPe.UI's Windows are set to be shown on a preset position on the screen:

• Center
• Center/Left
• Center/Right
• Top
• Top/Left
• Top/Right
• Bottom
• Bottom/Left
• Bottom/Right

So I didn't have, yet, a general use Window system where this could be applied. But the use case is valid, so I will keep this around until there.