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.
introduce new event type syntax domain:event for NodeConnection events
fix jshint errors
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 pull request #6942 from adobe/jasonsanjose/issue-6933

Introduce new event type syntax domain:event for NodeConnection events