Show line indicator instead of rectangle while dragging in tables (2.0)

[Nerixyz]

See #5252. This is essentially the same implementation, except that the TableRowDragStyle creates its own QStyle based on the currently applied style (which is the application style). Previously, the widget this style was applied to didn't have its own style, which causes it to default to the application style. And since QProxyStyle adopts the style without any agreement from the current parent (qApp), this causes a crash after deleting the proxy (and its children). I thought it would be a bit more involved (see message in #5255).

You can test how the objects are destroyed by breaking:

QObject::connect(proxyStyle, &QObject::destroyed, [] {
    _CrtDbgBreak();
});
QObject::connect(proxyStyle->baseStyle(), &QObject::destroyed, [] {
    _CrtDbgBreak();
});
QObject::connect(view, &QObject::destroyed, [] {
    _CrtDbgBreak();
});