Enable trojita to use System Proxy Settings

Underlying sockets can be made aware of the requirement of using System Proxy Settings.
Best match Proxy Settings are applied to the socket before connection is made.

REVIEW: 115429
Add proper credits for Karan
net: Add information about the used proxy to the network debug messages

Previously, there was no indication that a proxy was actually being used.

REVIEW: 116482
Add translations contexts to messages with format specifiers

REVIEW: 116493
More el6 build fixes

This is the cost of lazy developers (like me) and only an ex-post CI which
nobody bothers to check (ahain, like me).
platform detection: qt5: Use the QPA platform name instead of the removed Q_WS_* #defines
platform detection: Remove support for Symbian

This is dead code anyway, Trojita has never run there, AFAIK, and the platform
is now EOLed. When someone does the port, they can easily dig this from the
history.
platform detection: Recognize MacOSX 10.8

The docs for Qt 4.8 say that that the enum value is available in there; let's
trust that.
platform detection: Add support for Windows 8

The documentation says that this is *not* available on Qt 4.8, so it's a
Qt5-only feature for now.
Improve platform detection -- adding new platforms and support for Qt5

REVIEW: 116564
obs: adding .dsc for Ubuntu 13.10

These have been present in the OBS for a long time.
Removing obsolete files, that Ubuntu is EOLed
Adding proper credits for Luigi
Version bump to 0.4 "Україна"
tests: Support specifying the list of mailboex manually
tests: switch the DeleteMailbox tests over to LibMailboxSync fixture
tests: switch to the c*** API as well
Marking the current version as -git again
Explicitly mark libs as STATIC to not allow cmake rules override it.
www: link to the latest installer prepared by Christian Degenkolb

CCMAIL: christian@degenkolb.net