templates/windows/installer: Add command line flag for portable installation

Users of the Installer can now specify the `/Portable` flag in addition to other flags to install as Portable.

templates/windows/installer.iss.in

@@ -154,6 +154,24 @@ begin
 		PAnsiChar(S), SMTO_ABORTIFHUNG, 5000, MsgResult);
 end;
 
+// ------------------------------------------------------------------------------------------------------------------ //
+function ParamExists(Name: String): Boolean;
+var
+	Idx: Integer;
+	Limit: Integer;
+	Param: String;
+begin
+	Limit := ParamCount()
+	for Idx := 1 to Limit do begin
+		Param := ParamStr(Idx);
+		if SameText(Param, '/' + Name) then begin
+			Result := True;
+			exit;
+		end;
+	end;
+	Result := False;
+end;
+
 // ------------------------------------------------------------------------------------------------------------------ //
 var
 	oModePageSystemChoice: TNewRadioButton;
@@ -196,9 +214,15 @@ procedure OnModePageSystemChoiceClick(Sender: TObject); forward;
 // ------------------------------------------------------------------------------------------------------------------ //
 function InitializeSetup(): Boolean;
 begin
+	if ParamExists('Portable') then begin
+		bIsSystemMode := False;
+		bIsUserMode := False;
+		bIsPortableMode := True;
+	end else begin
 		bIsSystemMode := IsAdmin();
-	bIsUsermode := not IsAdmin();
+		bIsUserMode := not IsAdmin();
 		bIsPortableMode := False;
+	end;
 
 	Result := True;
 end;