[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.
qt: add transifex configuration file

This is a project-wide configuration file and should be the same for
everyone.
Also remove mention of creating it yourself from the translation process.
devtools: add a script to fetch and postprocess translations

Run this script from the root of the repository to update all translations from transifex.
It will do the following automatically:

- create a transifex configuration file
- fetch all translations
- post-process them into valid and committable format
Full translation update

Last update (48be9ce) missed quite a lot, for some reason.

This is also the first update done with the new script
`contrib/devtools/update-translations.py`
gitian: build against Qt 4.6

Should make it possible to run the resulting GUI executable on
Linux distributions that use Qt 4.6, such as Debian Wheezy and Tails.

Builds a mini-SDK for building against Qt 4.6. This includes the headers
as well as host utilities such as `lrelease`, `qrc` and `moc`.

This speeds up the gitian build a bit - libqt4-dev pulled in a lot of packages,
and is no longer needed as this provides a replacement of our own.

Note: This does not replace the Qt build with at static library. After this
commit we still build dynamically against the system Qt library. The only
difference is that compatibility with an older version is maintained. This
loses minor GUI functionality (such as setPlaceholderText) but still
allows integration into the window management of the host OS, unlike
when statically linking.
gitian: don't export any symbols from executable

This avoids conflicts between the libraries statically linked into bitcoin and any
libraries we may link dynamically (such as Qt and OpenSSL, see issue #4094).
It also avoids start-up overhead to not export any unnecessary symbols.
To do this, build a linker script that marks all symbols as local.
devtools: have symbol check script check for exported symbols

After last commit, our executables should export no symbols anymore.  To
make sure that this stays the case, verify this in the symbol checker
script.