v4.0.0: Composited-changed event & better settings

Breaking changes:

• --webkit-settings now takes the settings dash-separated (previously: underscore_separated). This puts them in line with what the properties are called in the underlying WebKitSettings object, which documentation is linked from the readme. You can see the available settings with --webkit-settings help.

Features:

• Your JS can now listen to a 'composited-changed' event. It is raised when the ability of your desktop to render transparency changes. You can use it to decide what to do when you accidentally close your compositor; a good idea might be just to close the window.
• Related to the above, calling window.close() now closes the overlay window and exits the hudkit process.

Fixes:

• Better compilation instructions, and dependencies list for Linux Mint. (Thanks to @DrinkFromTheCup for feedback.)
• The settings available via --webkit-settings are now read at run-time from your installed WebKit library. Previously, it was a hardcoded list. This is nice because it lets --webkit-settings help display allowed values and deprecation warnings, and the list of options will stay up-to-date with less effort as upstream WebKit developers add features.