Create editor context menu based on context

[RomanNikitenko]

Description

At the moment editor context menu is created only once and reused for all editors.
As result we have the same context menu for different editors:
https://youtu.be/kmZeuxAfe_I

As improvement editor context menu should be created based on current context on each call.
The issue based on #7149 (comment)

Reproduction Steps

Try to use editor context menu for different types of editors.
Pay attention on items and that some of them are disabled.
Also see the short video above.

OS and Theia version:
Fedora 31
Theia: 5fb75c7