Monitor files opened in DevStudio to detect changes from the outside

[crevoisier]

If a file opened in DevStudio is modified from outside, this file is not updated in DevStudio. Warn the user of a modification from outside and refresh this file.

[Arthoni]

Warn user if change detected on a file.
Strategy of resolution: keep content from devstudio / reload from disk / save as new [/ merge ?]

If implemented, should be an option, disabled by default.