Add SEP-07 transaction request support

[troggy]

What was done here:

• merged Add support for SEP-0007 pay and transaction requests satoshipay/solar#1202 (SEP-07 support) and Prepare optional installation of Solar as SEP-7 protocol handler satoshipay/solar#1098 (custom URI handler prompt)
• removed feature flags from the merged code (now showing Protocol Handler Permission and Trusted Services options all the time)
• registering handlers in electron app
• registering handler in web app on "Protocol Handler" settings option click
• added some translations (ES, IT, RU)

closes #46

[troggy]

The app should handle opening of web+stellar:tx and web+stellar:pay URIs now.

Should be working in the following environments:

• web (npm run dev in web/). Tested in Brave(Chromium) and Firefox
• electron app (npm run dev). Tested on MacOS only.. Please test on Windows and Linux
• android app (npm run build:android in cordova/). Tested in simulator and real device (Android 11)
• iphone app. Not tested

How to test:

• on the first run the app might ask you to install itself as a web+stellar protocol handler. Only for web and electron, no question on mobile device (by design)
• to test web+stellar:pay: generate payment request with https://eurmtl.me/seller/GCSAXEHZBQY65URLO6YYDOCTRLIGTNMGCQHVW2RZPFNPTEJN6VN7TFIN (payment goes to TFM, or replace with your address), click link on the bottom when testing on web/electron. Scan QR code when testing on mobile device. Example link: web+stellar:pay?destination=GCSAXEHZBQY65URLO6YYDOCTRLIGTNMGCQHVW2RZPFNPTEJN6VN7TFIN&amount=1&asset_code=EURMTL&asset_issuer=GACKTN5DAZGWXRWB2WLM6OPBDHAMT6SJNGLJZPQMEZBUR4JUGBX2UK7V&memo=Donate+from+Sunce&memo_type=MEMO_TEXT
• to test web+stellar:tx: create XDR with any operation for one of your wallets and type the following URI in a browser: web+stellar:tx?xdr=. Or use my example: web+stellar:tx?xdr=AAAAAgAAAADHzEa22QXTfQ7GJlrAvUPajqOTqBLMsfUGoXCr7Q7kjgAAAGQC/0Q8AAAABQAAAAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEAAAAAAAAACgAAAAR0ZXN0AAAAAQAAAAIxMQAAAAAAAAAAAAA= (you won't be able to sign though)