Feature/improve custom extension repos support (#540)

* Move "TextSetting" to new folder

* Extract dialog of "TextSetting"

* Immediately show dialog when adding a list item

* Reuse "TextSettingProps" for "MutableListSetting"

* Add "placeholder" prop to "MutableListSetting"

* Prevent duplicates by default

* Clear dialog values on save

* Validate repo url

src/i18n/locale/en.json

@@ -243,6 +243,11 @@
               }
             }
           },
+          "error": {
+            "label": {
+              "invalid_url": "Invalid repository url"
+            }
+          },
           "label": {
             "description": "Add custom repositories from which extensions can be installed",
             "title": "Custom repositories"
@@ -309,6 +314,7 @@
         "failed_to_save_changes": "Failed to save changes",
         "invalid_action": "This is not a valid action",
         "invalid_file_type": "Invalid filetype",
+        "invalid_input": "Invalid input",
         "update_failed": "Could not check for updates"
       }
     },