Fix bad cursor positioning when entering an inset

Fix bad cursor positioning when entering an inset

The test added at 359aef92 was incorrect. Actually, Inset::editXY
returns the inset when it is edited and also when it is not, which
can be confusing. So if we are unlucky and the slices positions in the
text area and in the inner inset match, the code below triggers and
many bad things can happen.

Also improve the documentation of Inset::editXY and TextMetrics::editXY.

Fixes #10691.