Clipboard-based keymap management

[nickcoutsos]

In order to support users who would prefer to compile locally I've introduced a new keymap "source". It's primarily a textbox where you can paste the contents of your keymap file but once you're set up there's a button to conveniently copy keymap changes to the clipboard. The bulk of the work to support this was backend refactors to support devicetree parsing in the browser, which is a big step toward supporting BLE updates whenever that functionality becomes available.

There are some gotchas. The typical limitations of the Keymap Editor apply here, but additionally there's no directory structure to browse so it can't "discover" layout data or board/shield data from which to generate a layout. This means that you'll need to manually provide that data when setting up. In the future you'll also be able to select one of the pre-generated layouts but I wanted to get these changes out the door quickly.

If you've given this feature a try please let me know how it worked out for you!