Unified UI and other tweaks

1.0 is coming, can you feel it? This PR brings many changes:

1. As discussed in #443 Zed now no longer has a separate project picker window. When you launch Zed, it will open a regular editor window with the project picker integrated (but all integrated into a single-window UI matching the editor theme). To open more Zed windows use the `Window:New` command (Command-Shift-N/Ctrl-Shift-N).
2. Native window chrome is back on Linux (Chrome app) and all standalone versions. Using the native window chrome makes Zed fit better with the rest of the OS. However, this has not (yet) been enabled for Windows and Mac in the Chrome app version because on those OSes the title bar isn't capable of showing text (particularly: the project name). This is rather important information, so I've opted to keep the custom windows chrome there for now.
3. Keys have been tweaked (see the keys.json in the diff) to be more consistent, and to work better on Linux and Windows (there were many issues there, as it turns out -- I recently became a Linux user).