[Qt] catch Windows shutdown events while client is running

- prevents unsafe shutdowns on Windows, which is known to be
  able to cause problems with wallet.dat
- if a users ends a Windows session, this will initiate a client shutdown
  and show a Windows dialog, that tells the user what is going on (for
  Windows Vista and higher it will even show a reason for blocking the
  Windows session end)
devtools: add script to check symbols from Linux gitian executables

Add a script to check that the (Linux) executables produced by gitian
only contain allowed gcc, glibc and libstdc++ version symbols.  This
makes sure they are still compatible with the minimum supported Linux
distribution versions.
Update contrib/debian for autotools, among a few other tweaks
Merge pull request #4098

6e40fcf Update contrib/debian for autotools, among a few other tweaks (Matt Corallo)
Merge pull request #4089

74fc254 devtools: add script to check symbols from Linux gitian executables (Wladimir J. van der Laan)
Merge pull request #4043

d282c1f [Qt] catch Windows shutdown events while client is running (Philip Kaufmann)
qt: Periodic translations update

Update English translation from source code, and pull other translations
from Transifex.