[Qt] replace fee slider with a Dropdown, extend conf. targets

Summary:
[Qt] replace fee slider with a Dropdown, extend conf. targets
[Qt] migrate old fee slider value to new dropbown Always round up (conservative)

Backport of Core PR10769
https://github.com/bitcoin/bitcoin/pull/10769/files
Completes T509

Test Plan:
make check
bitcoin-qt shows dropdown instead of slider as expected

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2406