Fixing Singleton typing to correctly return child class (#1914)

- Add distinction between `getInstance` and `getInstanceOrCreate` since
  it is not always possible to create an instance (since you might not
  know the correct arguments)

- Remove all the `export const *Store = *Store.getInstance<*Store>();`
  calls as it defeats the purpose of `Singleton`. Plus with the typing
  changes the appropriate `*Store.getInstance()` is "short enough".

- Special case the two extension export facades to not need to use
  `getInstanceOrCreate`. Plus since they are just facades it is always
  possible to create them.

- Move some other types to be also `Singleton`'s: ExtensionLoader,
  ExtensionDiscovery, ThemeStore, LocalizationStore, ...

- Fixed dev-run always using the same port with electron inspect

- Update Store documentation with new recommendations about creating
  instances of singletons

- Fix all unit tests to create their dependent singletons

Signed-off-by: Sebastian Malton <sebastian@malton.name>

src/renderer/components/select/select.tsx

@@ -8,7 +8,7 @@ import { observer } from "mobx-react";
 import { autobind, cssNames } from "../../utils";
 import ReactSelect, { ActionMeta, components, Props as ReactSelectProps, Styles } from "react-select";
 import Creatable, { CreatableProps } from "react-select/creatable";
-import { themeStore } from "../../theme.store";
+import { ThemeStore } from "../../theme.store";
 
 const { Menu } = components;
 
@@ -40,7 +40,7 @@ export class Select extends React.Component<SelectProps> {
   };
 
   @computed get theme() {
-    return this.props.themeName || themeStore.activeTheme.type;
+    return this.props.themeName || ThemeStore.getInstance().activeTheme.type;
   }
 
   private styles: Styles = {