property: add the necessary include file for std::function

This fixes the following error:

/home/aurel/devel/sigrok/pulseview/pv/prop/property.h:39:15: error: ‘function’ in namespace ‘std’ does not name a template type
  typedef std::function<GVariant* ()> Getter;
SigSession: Fixed std::thread joining
DecoderStack: Fixed thread dead-lock
.gitignore: Added new Qt compiler intermediate
Update #include directives to work with Qt5 as well as Qt4
Change QApplication::translate to tr, removing deprecated UTF8 parameter
INSTALL: Document C++11 compiler support requirement.
INSTALL: libboost-thread is no longer needed.
test/CMakeLists.txt: Drop obsolete boost-thread references.
Allow PulseView to be built with Qt5

cmake will auto-detect Qt4 or Qt5.  If both are available, Qt5 will be
selected unless FORCE_QT4 is set to TRUE.
CMakeLists.txt: Drop quotes causing invalid include paths for Qt5
Ignore Qt5-generated moc and qrc files.
Add missing files to test/CMakeLists.txt.
Use 'struct' for test cases defined with BOOST_AUTO_TEST_CASE.
CMakeLists.txt: Pass -DENABLE_SIGNALS to the compiler.

Until now only the cmake variable ENABLE_SIGNALS was used to add
signalhandler.{cpp,h} to the list of files to build.

However, main.cpp also uses '#ifdef ENABLE_SIGNALS' which didn't work
so far, since -DENABLE_SIGNALS was not passed to the compiler.

(one effect being that PulseView cannot be killed via CTRL-C from
an xterm or the like)

This fixes bug #368.
Fix compilation on 32-bit systems

storesession.h was declaring fields as std::atomic<uint64_t>, a type
not avaiable on 32-bit systems.  However, the values stored in these
fields were only used as input to QProgressDialog, which takes int,
not uint64_t.  So the fields could just as well be std::atomic<int>.

Also, added code to scale the progress values down if they would not
fit in an int.  (This would have been needed even if the fields were
to remain as uint64_t.)
Add missing #include <cassert> in multiple files
Declare a virtual destructor for SignalData

This fixes the following error:

error: deleting object of polymorphic class type 'pv::data::Analog'
which has non-virtual destructor might cause undefined behaviour
[-Werror=delete-non-virtual-dtor]

Also, the same error for pv::data::Logic.
CMakeLists.txt: Put quotes back for CXX_FLAGS only

Turns out these were needed after all.
MarginWidget: Disable system background

This fixes a rendering problem on Android.