Allow user-defined block explorers

[ghost]

In Settings/General Preferences the Bitcoin Explorer and Bisq Explorer may now be user defined in addition to the hard coded block explorer choices.

This implementation takes advantage of the existing Protobuf implementation that stores a BlockExplorer Name/UrlTx/UrlAddress for BTC and for BSQ.

A new popup dialog is implemented which allows editing of the explorer details (name, txUrl, and addressUrl). Any of the system-defined explorer details may be selected into the edit fields thus allowing them to be used as templates, customized or an entirely new explorer to be used.

If the user switches choice from a customized back to a pre-defined explorer then the customized settings are gone; this is because the Protobuf storage only allows saving one selection.

Fixes #4371
Closes #4634

[ghost]

Fixes #4634

[cd2357]

ACK, tested and works as expected.

[chimp1984]

utACK

Beside 2 nits... See comments

[chimp1984]

utACK

[ripcurlx]

NACK

Code looks fine, but please change the style of the >> button to match our general button style.

[sqrrm]

@ripcurlx Are you ok with this PR now? Will leave merge for you

[cbeams]

Great to see this functionality, thanks @jmacxx!

One note for potential future contributions: it would be great to be able to have more than one custom explorer. This is from my original description of the feature at #4371 (comment):

While it might be easiest to support adding just one custom block explorer, it would be ideal to allow specifying multiple of them, because in the case of an explorer running on one's local network, one might wish to access it via clearnet http when at home and via Tor when remote. This would require specifying two sets of URLs. And of course the user might want to specify any number of additional custom explorer services.