RFC: Progress bar widget

[viciious]

libRocket/libRocket#32

This looks potentially useful and viable for promotion to a stock widget but needs proper merging and a sample program.

[mikke89]

Looks useful to me. However I would implement it a bit differently. It is a bit inflexible and awkward to specify the images in attributes. Instead, I suggest doing it more like this:

• Generate an extra underlying (non-dom) element, kind of like we do in scrollbars, which represents the "filled" value.
• Now, we can easily use decorators as normal, we can have one for the background, and a separate one for the fill.
• Clip the fill element such that its width is proportional to its value. We could use an attribute to tell whether this should happen up/down/left/right.

This would be more flexible, allowing us to use any decorator including sprites to style both the background and fill, or just background-color if we like.

Also, it should be part of the controls library in my opinion.

As for the sample, I think we should create a demo sample that demonstrates all the controls (and possibly other features in general), which includes this.

[mikke89]

Implemented in 634543b.