[backport#15153] gui: Add Open Wallet menu

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/6c49a55b472fa4ec403e48ec718af85bfd402199

This is a partial backport of Core PR15153

Test Plan:

ninja check

./src/qt/bitcoin-qt -regtest -server
./src/bitcoin-cli -regtest createwallet test
./src/bitcoin-cli -regtest unloadwallet test

Changing focus to Open Wallet should reveal 'test', clicking it reveals combobox at right of interface allowing to switch between '[Default wallet]' and 'test'

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6115