support overwriting the label of detected tasks

[elaihau]

Description

It would be nice if overwriting the labels of detected tasks is supported in Theia.

Reproduction Steps

It is a feature in vs code. Users are able to replace the label of a detected task with a custom one by add "label" property in the config.

See GIF below to see how it works in vs code:

OS and Theia version:

Ubuntu 18.04. Example browser.

Version 0e16fe0

Diagnostics:

In theia, we don't handle the "label" property in TaskCustomization properly.

[lmcbout]

I also noticed that the task are being kept in the local storage. So when you restart the application, the "label" of the task remains even if you removed the label from the tasks.json

[tsmaeder]

@lmcbout that would be quite a different error than what this issue is about. Could you open a separate issue with steps to reproduce, please?

[lmcbout]

@tsmaeder I tried to reproduce the issue I reported last november with the latest Theia 1.0. I could not. If ever I can reproduce it, I will raise a new issue with steps to reproduce.