Adding a progress-bar widget

[Weibye]

Objective

• On our way to solve #5604 we need a lot of different widgets.
• This attempts to add a progress-bar widget
• Which can be used for loading indicators, health / mana-bars.

Solution

https://user-images.githubusercontent.com/13300393/200424470-89ad311c-fd54-482c-a984-5dab902c8929.mp4

• Same as #6236, the general design-approach here is "Widgets as lose collections of hierarchies of bundles".
• For the loading-bar widget to work, the user needs to create a NodeBundle with a ProgressBarWidget-component attached, that has a child NodeBundle with a ProgressBarInner-component.
• It is completely up to the user to make sure these requirements are met.

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Changelog

Added

• Progress bar widget

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Notes

• Please critique this general approach. I know how we can setup a lot of other widgets the same way, but I have not yet any clear answers for the bigger-picture problems such as "widget composition" and "widget styling".
• My gut feeling says it will be easier to tackle those problems once we have some real uses of various widgets in Bevy.

[alice-i-cecile]

@bzm3r review plz!

[Weibye]

widgets really feel like they belong in their own bevy_widgets crate?

We should also setup a new category of examples for widgets IMO. Should these two things happen in this PR, or somewhere else?

[alice-i-cecile]

I think in this PR: it's already going to be controversial, so let's do this right from the start.

[AxiomaticSemantics]

I'd like to see support for generics for min, max, progress values if that is reasonable. (Most of my use of progress bars are based on {u,i}N values)

This implementation should likely be coordinating with the proposed UI slider widget WRT internal representation of progress.

[Weibye]

I'd like to see support for generics for min, max, progress values if that is reasonable. (Most of my use of progress bars are based on {u,i}N values)

I'm not too versed in generic rust yet, so I'm not entirely sure how that would look but if you'd provide a code snippet of what that would look like, I'm happy to include it :)