BlockTip struct created and connected to notifyHeaderTip and notifyBlockTip signals.

Summary:
This is a backport of Core PR17993 [2/2]
https://github.com/bitcoin/bitcoin/pull/17993/commits/a06e845e826acaeb0db7cf02b2519c177e94dee5

Depends on D9253

Test Plan:
ninja && src/qt/bitcoin-qt
Send a transaction, wait for a few block and check that the tx status is correctly updated in the interface.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9254