QPrinter: constructor hangs when CUPS server or printer is unreachable

The QPrinter constructor hangs if the CUPS server, or a printer attempted accessed by the CUPS server, is unreachable. This may freeze the GUI, and should be avoided.

Following is the stack trace from starting Designer when a printer is unreachable from the CUPS server:

Program received signal SIGINT, Interrupt.
[Switching to Thread 0xb638a6d0 (LWP 4230)]
0xffffe430 in __kernel_vsyscall ()
(gdb) bt
#0 0xffffe430 in __kernel_vsyscall ()
#1 0xb6672e41 in connect () from /lib/libpthread.so.0
#2 0xb3febbdf in httpAddrConnect () from /usr/lib/libcups.so.2
#3 0xb3feaac6 in httpReconnect () from /usr/lib/libcups.so.2
#4 0xb3feb32c in httpConnectEncrypt () from /usr/lib/libcups.so.2
#5 0xb4001414 in cupsGetPPD2 () from /usr/lib/libcups.so.2
#6 0xb400162c in cupsGetPPD () from /usr/lib/libcups.so.2
#7 0xb70cbe56 in QCUPSSupport::setCurrentPrinter (this=0xbf97b9ec, index=4) at /home/sthomassen/dev/qt-44/src/gui/painting/qcups.cpp:158
#8 0xb70c5bdc in QPrinterInfo::availablePrinters () at /home/sthomassen/dev/qt-44/src/gui/painting/qprinterinfo_unix.cpp:797
#9 0xb70c5e91 in QPrinterInfo::defaultPrinter () at /home/sthomassen/dev/qt-44/src/gui/painting/qprinterinfo_unix.cpp:828
#10 0xb7039d41 in QPrinter (this=0x84e45ec, mode=QPrinter::HighResolution) at /home/sthomassen/dev/qt-44/src/gui/painting/qprinter.cpp:567
#11 0x0809e9e7 in QDesignerActions (this=0x84e4558, workbench=0x811ff80) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/qdesigner_actions.cpp:182
#12 0x0808136c in QDesignerWorkbench::initialize (this=0x811ff80) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/qdesigner_workbench.cpp:282
#13 0x080878ff in QDesignerWorkbench (this=0x811ff80) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/qdesigner_workbench.cpp:163
#14 0x08077d66 in QDesigner::initialize (this=0xbf97c398) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/qdesigner.cpp:182
#15 0x08078ba2 in QDesigner (this=0xbf97c398, argc=@0xbf97c400, argv=0xbf97c484) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/qdesigner.cpp:66
#16 0x08076b14 in main (argc=1, argv=0xbf97c484) at /home/sthomassen/dev/qt-44/tools/designer/src/designer/main.cpp:38