Simplify the logic of drag & drop handling to fix a bug.
Simplify the logic of drag & drop handling to fix a bug.

Crash log:
------------------------------------------
Termination Signal:    Segmentation fault: 11
Termination Reason:    Namespace SIGNAL, Code 0xb
Terminating Process:   exc handler [0]

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   org.qt-project.QtCore         	0x0000000104a15424 QSortFilterProxyModel::parent(QModelIndex const&) const + 84
1   org.qt-project.QtWidgets      	0x00000001017344d7 QTreeView::isIndexHidden(QModelIndex const&) const + 71
2   org.qt-project.QtWidgets      	0x000000010172853d QTreeView::visualRect(QModelIndex const&) const + 93
3   org.qt-project.QtWidgets      	0x00000001016e4d28 QAbstractItemView::update(QModelIndex const&) + 40
4   org.qt-project.QtWidgets      	0x00000001016f0855 QAbstractItemView::dataChanged(QModelIndex const&, QModelIndex const&, QVector<int> const&) + 645
5   com.seafile.seafile-client    	0x00000001007c9708 RepoTreeView::updateBackground() + 312 (repo-tree-view.cpp:926)
6   com.seafile.seafile-client    	0x00000001007c8e52 RepoTreeView::updateDropTarget(QModelIndex const&) + 482 (repo-tree-view.cpp:952)
7   com.seafile.seafile-client    	0x00000001007c91a0 RepoTreeView::dragMoveEvent(QDragM

haiwen/seafile-client,haiwen/seafile-client,haiwen/seafile-client,haiwen/seafile-client