Add proxy support for extension installation.

Configurable via a "proxy" preference (which could be used for other parts of
Brackets that should use HTTP proxy).

This fixes #6241, #5958 and [this backlog item](https://trello.com/c/SEV95fdK/902-support-for-a-proxy-in-extensions-installation)
Merge pull request #6911 from adobe/dangoor/extman-proxy

Add proxy support for extension installation.
add a read and compare on blind-write detection
rtc: better naming and added unit test
Merge pull request #6949 from adobe/jeff/6437

Fix spurious file write dialog
Merge branch 'master' into cmv4
Merge branch 'cmv4' into nj/cmv4-helper-unit-tests
Move doMultipleEdits() to Document