UI: Fix position of Sources dock actions on horizontal resize

Attempting to resize the Sources dock to be smaller than when initially
loaded would result in the contents of the list to never shrink.
Switching to another scene & back would temporary fix the sizing.

Fixes an issue introduced in adba393ca85fba19ed1bf6cd825ab8188beb2d16

UI/source-tree.cpp

@@ -1751,7 +1751,7 @@ SourceTreeDelegate::SourceTreeDelegate(QObject *parent)
 {
 }
 
-QSize SourceTreeDelegate::sizeHint(const QStyleOptionViewItem &,
+QSize SourceTreeDelegate::sizeHint(const QStyleOptionViewItem &option,
 				   const QModelIndex &index) const
 {
 	SourceTree *tree = qobject_cast<SourceTree *>(parent());
@@ -1760,5 +1760,5 @@ QSize SourceTreeDelegate::sizeHint(const QStyleOptionViewItem &,
 	if (!item)
 		return (QSize(0, 0));
 
-	return (QSize(item->width(), item->height()));
+	return (QSize(option.widget->minimumWidth(), item->height()));
 }