BREAKING CHANGES - Library Reform (see comment for details).

[mitchmindtree]

Sorry for the massive commit! The reform involved a lot of changes and it made sense to address some older issues while I was going. In general, I'm super excited about these changes :)

Here's a quick summary of changes before going into deets for a couple:

• Removed Drawable trait in favour of new .set method for deferred drawing (see below).
• Removed Callable trait and moved callback method to the widget impls to save on imports.
• Removed color.rs module in favour of PistonDevelopers/color crate.
• Fixed / vastly improved Positionable trait and removed annoying Ui parameter.
• Added methods for alignment (top, bottom, left, right, middle) i.e. .down(20.0).align_middle(). (the default vertical alignment is Left and the default horizontal alignment is Top.
• Switched graphics over to elmesque.
• Fixed issue where text would flow off widget.
• Deleted unnecessary dimensions.rs, rectangle.rs, shape.rs, draw.rs, and point.rs modules.
• Added a Widget struct to make the Uis widget_cache much cleaner.
• Inlined a few unnecessary widget module functions that were only being called in one place.
• Changed the macro-generated set_state function to ui.set_widget(ui_id, Widget {...}).

Removed Drawable trait in favour of .set method.

This change allows us to store renderable Elements in the Ui for deferred drawing, rather than drawing the widgets at their call site. This will allow ourselves and users to set custom rendering depths for widgets and will also make it extremely easy to make sure that captured widgets are rendered last (i.e. drawing an open DropDownList on top of widgets that are called after it).

Also, I've moved the UiId from the widget constructor to this set method so the user only has to deal with Ui related things in a single method. i.e. this

Button::new(ui_id).draw(ui, gl);

is now this

Button::new().set(ui_id, ui);

Now, draw is only called once: on the Ui type after all the widgets have been set. When called, it will sort the widgets by rendering depth and then render all of the stored elements.

ui.draw(gl);

Switched graphics over to elmesque

This is the change that made deferring the rendering of widgets possible - rather than having to directly draw the widgets at the widgets' call site, we can use elmesque to compose an Element and store it within our WidgetCache. Note that elmesque still uses the graphics crate under the hood, but offers a high level interface for composition and layout. One issue with this is that it will require more allocations in order to be able to store the data within the Elements and in turn this may impact performance. I have a few ideas for addressing this, but that will be for a future PR.

What next?

These changes open up a few doorways! These are what I'm hoping to implement over the next couple days:

• Register captured widgets within the Ui for consideration when sorting for rendering order.
• Scroll functionality for DropDownList.
• The Positionable trait is ready for easy integration with Canvases (see #380), which will be one of the final steps towards offering a fully featured layout API without the need for coords :)
• Unique per-widget theme-ing.

Closes #372, #350, #354, #374, #373, #347, #349, #218, #344 and #145 🎉

[bvssvni]

Nice! Now we can use Button as a object-as-function just like in piston-graphics!

[mitchmindtree]

@bvssvni has reviewed - there are a few things to address but they can easily be done in a future PR - going to merge 👍