Use callback to frontend extension Settings change to add/remove entries to launcher and main menu

[mahendrapaipuri]

We have added the configurability of text notebook entries in front end extension via JupyterLab Settings in #1163. But the current implementation needs users to refresh their browser tabs to take Settings changes into account.

We can improve it by firing a callback whenever Settings changes and modify the entries in launcher and main menu dynamically without having to refresh browser tab.