Usage of std::thread requires linking with appropriate system thread library.

This fixes the following link error (on debian sid, gcc 4.9):

Linking CXX executable pulseview
/usr/bin/ld: CMakeFiles/pulseview.dir/pv/sigsession.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
CMakeFiles/pulseview.dir/build.make:2844: recipe for target 'pulseview' failed
Use QApplication metadata for persistent storage

Since the application metadata is set in application.c,
we should use it instead of providing it manually every
time we create a QSettings object.

As I understand it, subclassing QApplication in application.c
is only a workaround for bad Qt behavior. In the event
that it is removed, the application metadata should be moved
so that persistent storage still works.
Fix the C++11 thread linking issues for unit tests as well.
Add serial_num/connection_id handling and save/restore last device

This patch performs several correlated changes:
- Make use of sdi->serial_num and sdi->connection_id in
format_device_title()
- Add a Device::get_device_info() method that returns a map
with all device identification information that's available
- Add a DeviceManager::find_device_from_info() method which
returns the best match to a given set of device infos
- Add save/restore code to the MainWindow class that makes
use of the aforementioned maps to identify the currently
selected device.