Add new `AdwButton` styles

[jesusrp98]

Closes #48 and #49.

This PR updates the AdwButton widget with new default styles, such as flat, pill and circle.

Here's a list of all changes implemented:

• Adapt default AdwButton constructor to represent the 'regular' adwaita button.
• Introduce AdwButton.circular style.
• Introduce AdwButton.pill style.
• Introduce AdwButton.flat style.
• Deprecate AdwTextButton in favor of AdwButton.
• Introduce child parameter on AdwButton, that is passed to the builder parameter (similar to other widgets in Flutter).
• Update animation duration and border radius parameters to coincide with libadwaita.
• Update all button state color to coincide with libadwaita.
• Add textStyle parameter to AdwButton.
• Simplify AdwHeaderButton and AdwSidebar widgets.

[MalcolmMielle]

I can have a look later tonight if you want me to @jesusrp98? Up to you.

[MalcolmMielle]

For the error given by the linter, I've open #51 because the super-linter ended up being quite annoying :S. I'm sorry, I know I introduced it, but I thought it would be nicer than it is :(.