TextArea: ensure selection handles are visible when inside the flickable

The current implementation would hide the handles when parented
into the flickable and the content view was flicked up. The handles
would in those cases have an y pos greater than the viewport height. But
since contentY would be greater as well, the handles should still
be visible.

This patch will ensure that the visible logic takes the handle parent
into account when calculating if it overlaps the viewport.

Change-Id: I591a4eb8a71b89720c80b190113888b5601fd060
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Fix TextField & TextArea mouse cursor shape

Mouse cursor shape is specified by an internal MouseArea inside editor.
Edit menu was placing another MouseArea on top, so the cursor shape was
lost. This change makes EditMenu re-use the same MouseArea.

Task-number: QTBUG-41955
Change-Id: I71e38bb42ea20f63b3e15c4c2cc7282b64b10d62
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>