GUI: refactor QComboBox initialisation in subframes
There is a lot of boilerplate code of the form,
_exp_combo->blockSignals(true);
QString current_exp = _exp_combo->currentText();
_exp_combo->clear();
if (gSession->experimentNames().empty())
return;
for (const QString& exp : gSession->experimentNames())
_exp_combo->addItem(exp);
_exp_combo->setCurrentText(current_exp);
_exp_combo->blockSignals(false);
for initialising combo boxes. This could be simplified by deriving subframes from a base class containing initialisation methods for these boxes, most of which appear in every subframe.