Test hover against the hit rect, not the bare frame rect

CreateBackdrop widens a frame's hit rect by the border, so mouse focus
and OnEnter/OnLeave cover a band that plain MouseIsOver reports as
outside. Add MouseIsOverHitRect, which passes the frame's hit rect
insets as MouseIsOver offsets, and use it at the unpadded call sites.
The XP bar's hover text follows the same OnEnter/OnLeave flag as its
fade.

modules/xpbar.lua

@@ -191,7 +191,7 @@ end
     local self = self or this
 
     if self.text_mouse == "1" then
-      self.bar.text:SetShown(MouseIsOver(self))
+      self.bar.text:SetShown(self.hover)
     end
 
     if self.always then return end