Make pre-Mojave tabs also show the close button on hover over tab

This makes them show up on tab hover, so you don't have to hover over
the exact location of the close button to see it.

src/MacVim/PSMTabBarControl/source/PSMYosemiteTabStyle.m

@@ -381,6 +381,7 @@ void YosemiteNSDrawWindowBackground(NSRect rect, NSColor *color)
         NSRect closeButtonRect = [cell closeButtonRectForFrame:cellFrame];
         NSImage *button = nil;
 
+        if ([cell isHighlighted]) button = closeButton;
         if ([cell closeButtonOver]) button = closeButtonOver;
         if ([cell closeButtonPressed]) button = closeButtonDown;