Fix check for root of content node tree

showContextMenu() is only called for content nodes from the mime part
tree, but the mime part tree has a separate copy of the node tree.
Therefore, isRoot was always false. Check content->isTopLevel() instead
of comparing pointers.
2 jobs for master in 17 minutes and 3 seconds (queued for 17 minutes and 32 seconds)
Status Name Job ID Coverage
  Build
canceled build_clazy_clang_tidy #103015

00:01:54

passed linux kf5-qt5 qt5.15 #103014

00:15:08

39.0%