Add dialog for transfer of BitShares tokens to BitShares EOS (BEOS) gateway

**Is your feature request related to a problem? Please describe.**
Add dialog to simplify creating a formatted memo for sending BitShares tokens to BEOS gateway. Currently in the reference wallet, a user must construct a fairly complex and potentially errorprone memo to instruct the gateway how to credit the BTS it receives.

**Describe the solution you'd like**
We have an existing implementation that solves this problem completely and it's been tested/operational for months now: https://github.com/bitshares/bitshares-ui/pull/2935

**Describe alternatives you've considered**
It has been suggested that some generic method could be designed for this to support many custodians. I'm sure this is possible, and even a good idea, but I don't think it should delay inclusion of the existing implementation that is tested and working well.

**Additional context**
There are screenshots in the PR.

Review the existing implementation in pull request #2935 and its screenshots first, since the issue names no source files or tests. Trace how the dialog creates the formatted memo for the BEOS gateway; done means users can create that memo without constructing it manually and the existing operational behavior is preserved.

Written by the indexing model from the issue text.