Scoped themes to apply only to the editor (#editor-holder)

https://github.com/adobe/brackets/issues/8415
https://github.com/adobe/brackets/issues/8420
The flickering was happening because I try to pre-emptively load a theme into the editor while themes are being loaded.  This is so that when a theme is removed, and added later, it can be reapplied when it is installed...  https://github.com/adobe/brackets/blob/master/src/view/ThemeManager.js#L309

So, that called refresh(true)...  Then because I subscribe my theme change listener so early, that would eventually get called, which would in turn call refresh(true)...  That double refresh true causes the flickering.  I had to be a little more strategic about setting up the listener so that I don't load the same theme twice when brackets is starting up.

I had done the alternative in the past of keeping track of an initted flag and what not, but that got pretty messy.  This was the cleanest and least intrusive change I came up with.