textview: Fix magnifier positioning/sizing nitpicks

The magnifier width is now set to at least 80px, and made to show at least
3 lines of text. The popover is made again to snap to line height, but is
shown closer to the selected line.

This can be squashed with 53f50f6e4cd9
textview: just show one line in magnifier
textview: raise a bit magnifier popover

Not a good hint if the finger covers bits of the magnifier itself, this
seems enough to at least only cover the tail.