qt: Change splash screen to normal window

Makes it possible to move, minimize, unminimize the window while
Bitcoin Core is initializing.
Also create pid file in non-daemon mode

Always make a pid file, not only when `-daemon` specified.

This is useful for troubleshooting, for attaching debuggers and loggers
and such.

- Write the pid file only after the datadir lock was acquired
- Don't create or remove a pid file on WIN32, and also don't show the option
Merge pull request #4941

a49f11d qt: Change splash screen to normal window (Wladimir J. van der Laan)
qt: Make splash and shutdown window ignore close events

It's strange to be able to close these windows while there is work
in progress.

Also set Qt::WA_DeleteOnClose on both windows to make sure that they
are deleted eventually, no matter what happens.
Merge pull request #4951

d6712db Also create pid file in non-daemon mode (Wladimir J. van der Laan)