Refactor to use gdk_threads_add_idle instead of gdk_threads_enter/leave

[trupples]

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[nunojsa]

LGTM... As @dNechita pointed out, we need to make sure it works on all supported OSes and to remove all the calls to gdk_threads_enter()/leave() also in the plugins (in a patch per plugin)

[trupples]

Tested on Ubuntu and Windows manually:

• context scan & select dialog behaves as expected
• fmcomms2_adv plugin changes seem to be on an unreachable code path anyway (starting with gtk signal do_fmcomms5_cal, which doesn't exist)
• fmcomms2_adv plugin otherwise looks and behaves as expected UI-wise, tested w/ Pluto

[dNechita]

• do_fmcomms5_cal

do_fmcomms5_cal is the name of button and is defined in the UI files (https://github.com/analogdevicesinc/iio-oscilloscope/blob/main/glade/fmcomms2_adv.glade#L6177).

[trupples]

Tested calibration UI with iio-emu generic pyadi-iio/test/emu/devices/fmcomms5.xml as a stand-in for an actual fmcomms5, and it seems to work ok. Couldn't get any samples to plot but I guess that's the setup's fault.