Editor file content synchronization and git checkout notification

[dkuleshov]

What does this PR do?

• Adds means to synchronize content of file opened in editor when it is being modified outside of che
• Adds means to track outside git check out operations (branch checkout or revision checkout) and inform a user

What issues does this PR fix or reference?

#1910
#1912

New Behavior

If file that is opened in che editor tab is modified somewhere else its content is updated
If git repository is checked out outside of che UI a user is notified about that

Tests written?

Yes

[codenvy-ci]

Build # 117 - FAILED

Please check console output at https://ci.codenvycorp.com/job/che-pullrequests-build/117/ to view the results.

[dkuleshov]

ci-build

[vparfonov]

ok

[codenvy-ci]

Build # 122 - FAILED

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Build # 148 - FAILED

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Build # 149 - FAILED

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Build # 150 - FAILED

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[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/168/

[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/179/

[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/181/

[codenvy-ci]

Build success. https://ci.codenvycorp.com/job/che-pullrequests-build/187/