test/CMakeLists.txt: Fix build on Windows/MinGW.

On Windows/MinGW we need to use Boost's 'thread_win32' instead of
'thread' (was already done in the main CMakeLists.txt, but not yet
in test/CMakeLists.txt).
Fixed AnalogSnapshotTest

This fixes bug #286
Add an extra sample to the logic edges so that the end sample is visible
pv::prop::Enum: Do not create a widget if the initial value could not be retrieved
Set default build type to RelWithDebInfo
pv::prop::Bool: Do not create a widget if the initial value could not be retrieved
pv::prop::Double: Do not create a widget if the initial value could not be retrieved
pv::prop::String: Do not create a widget if the initial value could not be retrieved
pv::prop::Int: Removed assert from Int::get_widget
Fixed off-by-one in DecoderStack::decode_proc
test/CMakeLists.txt: Fix MinGW build of the tests.
CMakeLists.txt, test/CMakeLists.txt: MinGW linking fix.
nsis: contrib/pulseview.nsi: Drop obsolete file.

We're only using the cross-compile version pulseview_cross.nsi these
days since it is much more convenient, and the old native build file
was not working and/or up-to-date anyway.