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.py

@@ -442,7 +442,7 @@ class DeveloperMode:
         if self.dialog.minPythonLineEdit.text():
             self.metadata.PythonMin = self.dialog.minPythonLineEdit.text()
         else:
-            self.metadata.PythonMin = "0.0.0" # Code for "unset"
+            self.metadata.PythonMin = "0.0.0"  # Code for "unset"
 
         # Content, people, and licenses should already be sync'ed
 
@@ -598,7 +598,7 @@ class DeveloperMode:
             )
             + "...\n"
         )
-        #pylint: disable=import-outside-toplevel
+        # pylint: disable=import-outside-toplevel
         import vermin
 
         required_minor_version = 0
@@ -640,10 +640,10 @@ class DeveloperMode:
 
     def _ensure_vermin_loaded(self) -> bool:
         try:
-            #pylint: disable=import-outside-toplevel,unused-import
+            # pylint: disable=import-outside-toplevel,unused-import
             import vermin
         except ImportError:
-            #pylint: disable=line-too-long
+            # pylint: disable=line-too-long
             response = QMessageBox.question(
                 self.dialog,
                 translate("AddonsInstaller", "Install Vermin?"),
@@ -695,7 +695,7 @@ class DeveloperMode:
                 )
                 return False
         try:
-            #pylint: disable=import-outside-toplevel
+            # pylint: disable=import-outside-toplevel
             import vermin
         except ImportError:
             response = QMessageBox.critical(