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.

editXY() should probably have a documented way to indicate whether the
cursor has entered the inset, although the test used right now is
probably OK. The inset value is useful mainly for context menus or
tasks that need to know which inset the (x, y) coordinates
point to.

Note finally that the documentation of TextMetrics::editXY is
incorrect concerning the return value. It will return a non-null value
if the cursor is on a not editable inset.

Fixes #10691.