* Widgets now get coloured when driven. The colour used is debatable, but I've just set it to a purplish colour so that it's easy to distinguish from animated/keyframes/highlight colours.
* New drivers are given generator F-Curve modifiers by default so that they at least show up in the view.
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.
