core: switch from mousedown to drag functions

Due to crashes/freezes while moving some frames, one possible
reason could be that pfUI was using OnMouseDown and OnMouseUp
to run its frame-moving code. I have noticed better performance
when using the intended functions, OnDragStart and OnDragStop
to move frames.

This commit changes every occurrence of such functions and splits
drag and click into seperate functions where required.

api/api.lua

@@ -526,8 +526,8 @@ end
 -- 'name'       [frame/string]  Name of the Frame that should be movable
 -- 'addon'      [string]        Addon that must be loaded before being able to access the frame
 -- 'blacklist'  [table]         A list of frames that should be deactivated for mouse usage
-local function MoveMouseDown() this:StartMoving() end
-local function MoveMouseUp() this:StopMovingOrSizing() end
+local function OnDragStart() this:StartMoving() end
+local function OnDragStop() this:StopMovingOrSizing() end
 function pfUI.api.EnableMovable(name, addon, blacklist)
   if addon then
     local scan = CreateFrame("Frame")
@@ -544,8 +544,9 @@ function pfUI.api.EnableMovable(name, addon, blacklist)
 
         frame:SetMovable(true)
         frame:EnableMouse(true)
-        frame:SetScript("OnMouseDown", MoveMouseDown)
-        frame:SetScript("OnMouseUp", MoveMouseUp)
+        frame:RegisterForDrag("LeftButton")
+        frame:SetScript("OnDragStart", OnDragStart)
+        frame:SetScript("OnDragStop", OnDragStop)
 
         this:UnregisterAllEvents()
       end
@@ -561,8 +562,9 @@ function pfUI.api.EnableMovable(name, addon, blacklist)
     if type(name) == "string" then frame = _G[name] end
     frame:SetMovable(true)
     frame:EnableMouse(true)
-    frame:SetScript("OnMouseDown", MoveMouseDown)
-    frame:SetScript("OnMouseUp", MoveMouseUp)
+    frame:RegisterForDrag("LeftButton")
+    frame:SetScript("OnDragStart", OnDragStart)
+    frame:SetScript("OnDragStop", OnDragStop)
   end
 end