qt: Wrap tooltips in the intro window

Summary: This is a backport of core-gui#20

Test Plan:
Open the intro window and check that the tooltips are wrapped (hover the cursor on the "enable pruning" checkbox)
src/qt/bitcoin-qt --regtest -choosedatadir

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10676