Separate out composer window management into a separate class

[jonathontoon]

Things are getting really messy in main.dev.js, should abstract some of it out similar to the OAuthManager class.

Expected Behavior

Everything should be easily read and understood without a huge file to scroll through.

Current Behavior

Everything is just dumped into one file. It works, but is awfully messy.

Possible Solution

Move the management of the composer browser window to it's own class.

Context

Makes debugging and development slow.