Add TextLayout core provider, remove / migrate old text provider implementation

[misl6]

RTL, BiDi, justification, line height, letter spacing, emojis, font fallback, ellipses, line breaking, max lines ...
Oh, what a mess.

Handling text and layouting it correctly, taking care of the needs of all the different scripts and languages from all around the world is not an easy task. But is something we certainly need to have to grow as a framework, and support all the communities around the world.

Over the years, we stratified a pile of workarounds on top of our Text core provider, docs, and unofficial guides with different levels of success. But, was not straightforward, not well integrated, and our users were expecting these features just out-of-the box.

The TextLayout core provider will offer an abstraction that can be used as a bridge between the TextLayout implementation and the widgets that need to lay out a text.

A TextLayout implementation can decide (and advertise) to not conform to all the methods. This will make it possible for our users to keep using non-fully compliant implementations if the fully compliant one is too huge for their use case, or not available on their target platform.

ATM (will update this issue as things evolve) , we're investigating using Skia/SkParagraph as the default TextLayout implementation.

Related issues to consider:

• Manage Fonts list and character recognition fallback in a platform independent manner. #1701
• Texture size too large causes invisible text #2119
• Issues with printing languages in unicode. #2669
• Can't type special characters into TextInput #3459
• TextInput rendering certain characters as boxes #4741
• Use Pango as text renderer #4902
• Memory leak in SDL2 text provider #5660
• Inconsistent Font Sizing between 1.9.1 and 1.10.1.dev0 (Retina Displays) #5684
• Vertically Centered Text - Last Line May or May Not Render #6385
• CJK support #6790
• Wrapping a single character in markup causes a line break. #7496
• Allow Original color of emoji font in Label #7565
• Arabic and RTL support #7704
• Textinput does not render emojis correctly #7995
• Sinhala letters are not complete in some places in kivy Label and sinhala font in kivy TextInput is wrong #8182
• label widgets breaks on text with lines more than 950 ie. no text is rendered #8213
• Document colored emoji feature, and how to use it #8319
• Add support for letter-spacing to Kivy #8395
• Add support for font-weight and variable fonts to Kivy #8394
• Line-height is not taken in to account in one liner text #8398
• Support international languages #8444