osc.c, oscplot.c: Change priority of timeout functions(capture_proccess() and plot_redraw()) to G_PRIORITY_DEFAULT_IDLE.
Reason: These timeout functions need to have lower priority than redrawing operations of GTK. Otherwise when capturing large amount of data the ploting never occurs.
oscplot: Fix bug: Comboboxtext of the fft size of a transform was crashing after changing the sample of the device.
Upon tranform creation, set the default size of the transform equal to the sample count of the device.