Clean up popup positioning in GUI

[HebaruSan]

Background

If a KSP update happens, CKAN will alert you at launch:

This window can also be accessed via the menus, Settings ⇒ Compatible KSP Versions.

Problems

• If it auto-opens at launch, this window will be positioned at the upper left corner of your leftmost monitor, which makes it difficult to notice
• A Save button normally saves something without closing, but this one closes the window
• Capitalization of "Ksp" in the title is wrong because it's an acronym standing for Kerbal Space Program
• If the user opens this window manually and then cancels out, the mod list is regenerated needlessly

Changes

• The window will be centered on the screen at startup
• The window will be centered on its parent if the user opens it manually
• The Save button now says Accept instead, to make it clearer that it closes the window
• "KSP" is now all caps
• There's a Close X button in the toolbar
• The mod list is only regenerated if the user clicks Accept

Other changes:

• The Select KSP Install window will now be centered on screen at startup
• The Select KSP Install window will now be centered on its parent if the user opens it manually