use new style connect, i.e. function pointers (#682)

use new style connect, i.e. function pointers (#682)

* migrate from old style connects to the new style.

The changes using qOverload were done by hand, the rest were done
with clazy 1.9.

The ones using the TreeAction constructor were not found by clazy
and would require more manual intervention.

Also, I note uic generates old style connects.

* eliminate our TreeAction class in favor of QMenu::addAction method.

It is a mystery to me why we created TreeAction.  The QMenu method existed
in Qt 4.3.  Documentation before 4.3 is scarce.
QMenu::addAction(const QString &text, const QObject *receiver, const char *member, const QKeySequence &shortcut = 0)

* clazy updates gmapdlg connects.

* add TODOs for Qt6 related to qOverload.