AddonManager: Consolidate preference in user.cfg from Plugins to Addons

Currently when a user first uses the Addon Manager a dialog popup
prompts them about how FC is not liable for the code plugin authors
provide. It stores the binary response in a `readWarning` boolean
within the user.cfg file.

This commit changes the location of said boolean from
`User parameter:Plugins` to `User parameter:Addons`.

It also migrates old the old setting if available.

Co-authored-by: Benjamin Nauck <benjamin@nauck.se>
Add suggestions from @chennes

Co-authored-by: Chris Hennes <chennes@pioneerlibrarysystem.org>

src/Mod/AddonManager/AddonManager.py

@@ -72,8 +72,10 @@ class CommandAddonManager:
     def Activated(self):
 
         # display first use dialog if needed
-        readWarning = FreeCAD.ParamGet("User parameter:Plugins/addonsRepository").GetBool("readWarning",
-                                                                                          False)
+        readWarningParameter = FreeCAD.ParamGet("User parameter:BaseApp/Preferences/Addons")
+        readWarning = readWarningParameter.GetBool("readWarning", False)
+        newReadWarningParameter = FreeCAD.ParamGet("User parameter:Plugins/addonsRepository")
+        readWarning |= newReadWarningParameter.GetBool("readWarning", False)
         if not readWarning:
             if (QtGui.QMessageBox.warning(None,
                                           "FreeCAD",
@@ -85,8 +87,7 @@ class CommandAddonManager:
                                           QtGui.QMessageBox.Cancel |
                                           QtGui.QMessageBox.Ok) !=
                     QtGui.QMessageBox.StandardButton.Cancel):
-                FreeCAD.ParamGet("User parameter:Plugins/addonsRepository").SetBool("readWarning",
-                                                                                    True)
+                readWarningParameter.SetBool("readWarning", True)
                 readWarning = True
 
         if readWarning: