Merge #15149: gui: Show current wallet name in window title

Summary:
fe7048b39 gui: Show current wallet name in window title (João Barbosa)
8a7926112 gui: Keep network style in BitcoinGUI (João Barbosa)
f411c8b35 gui: Remove unused return type in some BitcoinGUI methods (João Barbosa)

Pull request description:

<img width="876" alt="screenshot 2019-01-11 at 23 58 26" src="https://user-images.githubusercontent.com/3534524/51065458-d7ebaf80-15fc-11e9-9162-e37e9a10d448.png">

Tree-SHA512: 5c43f615834983bc1c5045e07c6e119044dd78ca947fd2679d302b519d5ce1d08d29ca00b1c11e88c4bbc4d56f2e6f4a8adc42084f3503e751e642e8a13112dc

Backport of Core PR15149
https://github.com/bitcoin/bitcoin/pull/15149/files

Depends on D4567
Completes T699

Test Plan:
ninja check
Verify bitcoin-qt shows wallet name in window title.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4568