Added stub of peak-hold
Merge branch 'master' into peak-hold

Conflicts:
	applications/gqrx/mainwindow.cpp
	applications/gqrx/mainwindow.h
	qtgui/dockfft.cpp
	qtgui/dockfft.h
	qtgui/dockfft.ui
	qtgui/plotter.cpp
	qtgui/plotter.h
Merge pull request #124 from Steven--/peak-hold

Adds peak hold to the FFT plot.