Make Mini-cart settings independent from cart

[achyuthajoy]

Description

At the moment, the PPEC on Mini-cart is controlled by the cart option. There are no specific options to disable Mini-cart PPEC and requires PPEC on cart to be enabled to show up.

This PR adds the following changes.

1. Re-purposes existing button settings as Global Settings, which takes effect if context-specific configuration is not defined.
2. Added context-specific configuration for Cart
3. Added option to disabled PPEC on Mini-cart
4. Re-purposes context-specific Mini-cart settings to show up only if PPEC on Mini-cart is enabled

Tested the changes on a new site & existing site that already uses PPEC. Couldn't find any compatibility issues and noticed that the existing settings are carried over.

Testing Changes

• [ ] Compare the PR against a new WooCommerce installation with the WordPress.org repository version to see if the default options are set correctly. PR will check enable mini-cart by default since there was no option to turn off PPEC on Mini-cart previously.
• [ ] On an existing site that uses PPEC, remove the plugin and use the development version to see if the previous settings are carried over.
• [ ] Test context-specific settings for Single Product / Mini-cart / Cart / Checkout along with the option to disable PPEC on specific pages (especially Mini-cart since it's a new feature).

Question

Since Mini-cart is enabled based on cart settings, on existing sites when the plugin is updated to include this PR, "Enable PayPal Checkout on Mini-cart" (default field value set to "yes") doesn't take effect if the settings are not saved.

Should we add an admin notice to save the settings once the plugin is updated? Or set the value of "Enable PayPal Checkout on Mini-cart" to "no" by default?

Not sure about documenting the new Global Settings & Mini-cart sections.

Closes #745.