Mobile: BottomSheet design tweaks

[etoledom]

Description

This PR adds some design options to the BottomSheet and its Cells.

• A new option for a cell with just one Label with centered text.
• A possibility to add custom styles to both Label and Value text.
• A new option for adding an icon to the cell.
• A possibility to completely hide the BottomSheet header.
• A new separator with an option for not showing it (helpful for the last cell).

To test:

• Checkout the related gutenberg-mobile PR: (wordpress-mobile/gutenberg-mobile#533)
• Run the example project.
• Tap on a image block with image.
• Tap on the image settings button (the wheel icon in the inline toolbar).
• Check that the BottomSheet displays as shown in the screenshot (with just one Alt text cell).
• If you feel like it, play with different configurations for the BottomSheet and the Cells.

[marecar3]

LGTM!

[marecar3]

LGTM!

[etoledom]

Thank you!