Update all eCash related references to more generic names

[xecdev]

This PR refactors the codebase to replace all eCash-specific references (including variable names, CSS class names, and general mentions) with more generic terms. This refactoring not only improves code quality but also lays the groundwork for supporting additional cryptocurrencies like BCH, DOGE, and BTC.

Key Changes:

• Refactoring: All eCash-related identifiers have been renamed to generic terms.
• Migration Plan: A detailed migration strategy is implemented to ensure existing plugin users keep their data. For example, the database column ecash_address in the wp_paybutton_paywall_unlocked table is now renamed to pb_paywall_user_wallet_address.
• Bug Fixes: Minor bugs introduced during the renaming process across files have been resolved.

Test plan:
1. Existing WordPress Site:

• Install the plugin on your existing WP site
• Make sure your old options such as Admin wallet address is still there
• Open the database (via the WP Adminer plugin) and hover over to the wp_paybutton_paywall_unlocked table, select the data and see if the column ecash_address name has changed to pb_paywall_user_wallet_address
• Go to the Customers page from dashboard and confirm that the data is loaded correctly
• Test the paywall functionality of the plugin from the front-end and make sure it works correctly

2. New WordPress Site:

• Install the plugin on a fresh WP installation to confirm that it functions correctly from the start.

[Klakurka]

Comments explaining the 'what' are usually pretty useless and this is a good example. Comments should cover the 'why' and then the code itself should be relatively easy to read.