gui, lib/config: Add default path for new folders (fixes #2157)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4192

lib/config/config_test.go

@@ -73,6 +73,7 @@ func TestDefaultValues(t *testing.T) {
 		KCPSendWindowSize:       128,
 		KCPUpdateIntervalMs:     25,
 		KCPFastResend:           false,
+		DefaultFolderPath:       "~",
 	}
 
 	cfg := New(device1)
@@ -221,6 +222,7 @@ func TestOverriddenValues(t *testing.T) {
 		KCPSendWindowSize:       1280,
 		KCPUpdateIntervalMs:     1000,
 		KCPFastResend:           true,
+		DefaultFolderPath:       "/media/syncthing",
 	}
 
 	os.Unsetenv("STNOUPGRADE")