Modify theme services to use preference service

[colin-grant-work]

What it does

This PR's primary objective is to modify the ThemeService and IconThemeService to interact directly with the PreferenceService as their primary source of truth. Previously, neither service was injectable, and so neither had access to the PreferenceService directly. As a consequence, their interactions with the PreferenceService were mediated by the CommonFrontendContribution and both services relied heavily on local storage for persistence.

How to test

1. Open your user settings.json and execute one of the commands Preferences: Color Theme or Preferences: Icon Theme.
2. Move the selector to preview themes.
3. Note that the settings.json file is not modified.
4. Select a theme from the quick pick.
5. Note that your settings.json is modified.
6. In the settings.json file, set a nonsensical value ("Definitely not any kind of theme!") for the preference.
7. Note that the theme you selected remains in place.
8. Reload the page.
9. Note that the theme you selected (if it was a color theme - we don't store assets of icon themes) still remains in place.

This is different from VSCode's behavior where the theme is reset to default on restart. It would be possible to implement this behavior, if preferable.

---

Check for #10938

1. Remove the Material Icon Theme from your plugins, if you have it.
2. Open the application, and in your user settings.json, set workbench.iconTheme to "material-icon-theme".
3. No change should be immediately observable.
4. Install the Material Icon Theme from Open VSX.
5. The theme should immediately take effect.

---

1. Set your theme to Red or something else with a distinctive editor background color.
2. Reload the application.
3. There should be no theme flashes.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍

• Selected themes (color/icon) are persisted across reloads
  • When changing the theme and reloading the app, the background color is correctly set to the last theme applied.
  • Even if the user enters an incorrect/nonsensical entry in the user settings
• Setting the icon theme and only loading the corresponding vscode extension afterwards correctly applies the icon theme to the application.

[307god]

let theiaDidInitialize = false;
const originalInitialize = StandaloneServices.initialize;
StandaloneServices.initialize = overrides => {
    if (!theiaDidInitialize) {
        console.warn('Monaco was initialized before overrides were installed by Theia\'s initialization.'
            + ' Please check the lifecycle of services that use Monaco and ensure that Monaco entities are not instantiated before Theia is initialized.', new Error());
    }
    return originalInitialize(overrides);
};

this contaminate @Injectable() MonacoFrontendApplicationContribution
if i want rebind MonacoFrontendApplicationContribution, i can`t change theiaDidInitialize and StandaloneServices.initialize,it always tell me 'Monaco was initialized before overrides were installed by Theia's initialization.'
what can i do?
@colin-grant-work @msujew