Change the sort algorithm to use UnitExist instead of frame:IsShown

When UNIT_PET fires, pet units will exist but the UnitWatch will not yet
have shown the frame.

src/types/unitgroup.lua

@@ -32,9 +32,10 @@ local function RoleValue(role)
 end
 
 local function UnitFrameDefaultCompare(lhs, rhs)
-    -- visible < invisible
-    if lhs.secureFrame:IsShown() ~= rhs.secureFrame:IsShown() then
-        return lhs.secureFrame:IsShown()
+    -- exists < not exists
+    local lexists, rexists = UnitExists(lhs.unit), UnitExists(rhs.unit)
+    if lexists ~= rexists then
+        return lexists
     end
 
     local lunit, runit = lhs.unit, rhs.unit
@@ -69,7 +70,6 @@ function UnitGroup:Init(left, top, width, height)
     self.top = top
     self.left = left
     self.units = {}
-    self.sortAfterCombat = false
     self:RegisterEvents()
 end
 
@@ -104,18 +104,14 @@ function UnitGroup:PLAYER_REGEN_ENABLED()
 end
 
 function UnitGroup:OnLeaveCombat()
-    if self.sortAfterCombat then
-        self:Sort()
-        self.sortAfterCombat = false
-    end
+    self:Sort()
 end
 
 function UnitGroup:OnRosterUpdate()
     if InCombatLockdown() then
-        self.sortAfterCombat = true
-    else
-        self:Sort()
+        return
     end
+    self:Sort()
 end
 
 function UnitGroup:AddUnitFrame(unit)