Improve implementation of TabWorkArea::posIsTab()

Improve implementation of TabWorkArea::posIsTab()

The Qt documentation states that tabAt() returns -1 if the position
is not over a tab. This behavior has been consistent since Qt 4.3
[1]. This commit's improvement likely makes the code faster in two
ways:

(1) we do not need to loop through potentially all tabs
(2) we only need to look up the tab index corresponding with one
position

posIsTab() is not currently used intensively so no practical gain in
speed is achieved, but it protects against future use.

[1] https://doc.qt.io/archives/4.3/qtabbar.html#tabAt