Use custom Welcome to Bitcoin Core dialog again

[BenWestgate]

Is your feature request related to a problem? Please describe.
Some users changed the prune setting to a suboptimal one. Code is difficult to work with because of the need to confirm they chose the correct datadir, if they walked away from the PC, the sync won't start until they click OK in the Welcome to Bitcoin Core dialog.

Describe the solution you'd like
Create our own dialog to convey the same info, see comments in #162 for screenshots.

Describe alternatives you've considered
Give them no Welcome at all, but this could lead to not understanding they can restart bitcoin during sync or that it will work their PC hard. Or if pruning was chosen by Bails that it limits how old of wallets can be loaded without repeating IBD.