Since the `client.{focused,focused_inactive,unfocused,urgent}` commands
change colors, the textures need to be updated otherwise the textures
and the rest of the title bar may utilize different colors.
Implements rendering of borders. Title text is still to do.
Implements the following configuration directives:
* client.focused
* client.focused_inactive
* client.unfocused
* client.urgent
* border
* default_border
