Add small utility modal to convert between address formats

[gonzamontiel]

For different testing purposes, we could use a tool for converting between different address formats, like stellar encoding, SS58, hexa, etc.
Add a button in the topbar, that shows a simple modal with an input for converting among different address formats.
Design TBD.

[TorstenStueber]

Please note that the conversion of binary Stellar keys to string encoded Stellar keys should never be required in the future. The extrinsic should just accept a string, which is the Stellar encoding. But this is good for now.