[backport#15153] gui: Show indeterminate progress dialog while opening wallet

Summary:
https://github.com/bitcoin/bitcoin/pull/15153/commits/1951ea4342db4122032660139248b79a02c574f3

Depends on D6128

This is a partial backport of Core PR15153

Test Plan:

ninja check

Run:

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

Choose: File -> Open Wallet -> test

See a modal with "Opening wallet test" pops up then disappears

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6129