UI: Clear ListItem selection before deleting widgets

This works around a crash in the "widget->clear()" call in
ClearListItems under the following circumstances:
- Create at least two scenes
- Create at least one source in both scenes
- Have at least one source selected in both scenes
- Set the same "Switch to scene" hotkey on both scenes
- Use "Switch to scene" hotkey

Reduced stack trace on my machine:
    frame #0: 0x00000001004bac2d QtWidgets`QWidget::show() + 93
    frame #1: 0x00000001006d32a2 QtWidgets`QAbstractItemView::updateEditorGeometries() + 690
    frame #2: 0x00000001006d36d7 QtWidgets`QAbstractItemView::updateGeometries() + 23
    frame #3: 0x00000001006f1ae6 QtWidgets`QListView::updateGeometries() + 438
    frame #4: 0x00000001006ccdce QtWidgets`QAbstractItemView::doItemsLayout() + 46
    frame #5: 0x00000001006f1916 QtWidgets`QListView::doItemsLayout() + 214
    frame #6: 0x00000001006f9cf3 QtWidgets`QListViewPrivate::rectForIndex(QModelIndex const&) const + 611
    frame #7: 0x00000001006eb48d QtWidgets`QListView::visualRect(QModelIndex const&) const + 29
    frame #8: 0x00000001006f1567 QtWidgets`QListView::visualRegionForSelection(QItemSelection const&) const + 1863
    frame #9: 0x00000001006d7ba9 QtWidgets`QAbstractItemView::selectionChanged(QItemSelection const&, QItemSelection const&) + 73
    frame #10: 0x00000001006f97d2 QtWidgets`QListView::selectionChanged(QItemSelection const&, QItemSelection const&) + 674
    frame #11: 0x00000001006d9b9e QtWidgets`QAbstractItemView::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) + 1246
    frame #12: 0x0000000102b10372 QtCore`QMetaObject::activate(QObject*, int, int, void**) + 2994
    frame #13: 0x0000000102aa001c QtCore`QItemSelectionModel::emitSelectionChanged(QItemSelection const&, QItemSelection const&) + 300
    frame #14: 0x0000000102a9fe6c QtCore`QItemSelectionModel::select(QItemSelection const&, QFlags<QItemSelectionModel::SelectionFlag>) + 748
    frame #15: 0x0000000102aa048b QtCore`QItemSelectionModel::clear() + 75
    frame #16: 0x000000010072e9d8 QtWidgets`QListWidget::clear() + 24
    frame #17: 0x000000010015759d obs`ClearListItems(widget=0x000000010683afa0) + 141 at item-widget-helpers.cpp:43
    frame #18: 0x000000010003424b obs`OBSBasic::UpdateSources(this=0x0000000105f5bf50, scene=(val = obs_scene * = 0x0000000110679440)), &(obs_scene_release)>) + 75 at window-basic-main.cpp:1254
    frame #19: 0x0000000100036a96 obs`OBSBasic::UpdateSceneSelection(this=0x0000000105f5bf50, source=<unavailable>), &(obs_source_release)>) + 422 at window-basic-main.cpp:1473
    frame #20: 0x0000000100175c48 obs`OBSBasic::qt_static_metacall(_o=0x0000000105f5bf50, _c=InvokeMetaMethod, _id=17, _a=0x00007fff5fbfb640) + 776 at moc_window-basic-main.cpp:494

This crash was reported at
https://obsproject.com/mantis/view.php?id=364

obs/item-widget-helpers.cpp

@@ -37,6 +37,8 @@ void DeleteListItem(QListWidget *widget, QListWidgetItem *item)
 
 void ClearListItems(QListWidget *widget)
 {
+	widget->setCurrentItem(nullptr, QItemSelectionModel::Clear);
+
 	for (int i = 0; i < widget->count(); i++)
 		delete widget->itemWidget(widget->item(i));