Addon Manager: Refactor installation code

Improve testability of installation code by refactoring it to completely
separate the GUI and non-GUI code, and to provide more robust support
for non-GUI access to some type of Addon Manager activity.

src/Mod/AddonManager/addonmanager_devmode_add_content.py

@@ -364,7 +364,7 @@ class AddContent:
         if not self.metadata:
             self.metadata = FreeCAD.Metadata()
         dlg = EditDependencies()
-        dlg.exec(self.metadata) # Modifies metadata directly
+        dlg.exec(self.metadata)  # Modifies metadata directly
 
 
 class EditTags:
@@ -536,7 +536,6 @@ class EditDependency:
 
         self.dialog.layout().setSizeConstraint(QLayout.SetFixedSize)
 
-
     def exec(
         self, dep_type="", dep_name="", dep_optional=False
     ) -> Tuple[str, str, bool]: