Simplify styling

[bundyo]

• Add PickerField element selector
• Add PickerPage element that inherits from Page in the modal and a selector for it
• Move the class set on ActionBar and ListView to the aforementioned PickerPage
• Document it

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing

What is the current behavior?

Currently the PickerField inherits all TextField styling, but cannot be targeted in CSS with an element selector. In addition, currently the modal page elements can be only targeted with a class set on the PickerField which gets transferred on the ActionBar and ListView. We need a way to directly style all instances of PickeField modals without the need to set a class.

What is the new behavior?

PickerField element selector directly targets the element. A new element selector PickerPage is set on the modal, which can be used to target the ActionBar and ListView in it. Also the class is now set on the PickerPage, instead on the elements in it.

Closes #11.

BREAKING CHANGES:

• Moves the classes from the ActionBar and ListView to the PickerPage element

Migration:

• Fix selectors from ListView.class to .class ListView.

[VladimirAmiorkov]

Great PR, all is left is to update the demo, demo-angular and demo-vue with the breaking changes.