Fix crash when clicking the preview icon.

For some reason the NSPanel was autoreleased after being loaded from the
nib. Fixed it by using `[NSWindowController initWithWindowNibName:]` instead
of loading the nib by hand in `mainWindowInBundle:`.
Upgrade PluginWindow.xib to Xcode 5 format

Note that this will prevent the xib to be edited in Xcode 4.x
Link the window outlet the file's owner (ATZPluginWindowController)
Merge pull request #80 from guillaume-algis/issue79-preview-icon-crash

Fix crash when clicking the preview icon. - Related to issue #79