Drivers can now be Added/Removed from buttons using the D/Alt-D hotkeys, and also through the menu. Driver settings (i.e. the target) are not set by default. To set those, go to the Graph Editor (see notes). Notes: * Buildsystem maintainers - I've added a new file "editors/animation/drivers.c" * Widget colours for the driven-setting indications are needed * To see the new drivers, go into Graph Editor -> "Drivers" mode. Currently, there's a little bug there which prevents editing of the new drivers.