Finding the dimensions of hidden textareas (Bug #1382159)

If the parents of a textarea are also hidden it returns height/width
as 0, which is no use to us. So we need to find it's height/width when
not hidden.

So added some code to:
- to clone the textarea
- make it positioned way outside the viewport
- append it to body
- find height/width
- remove the clone from body
- return the height/width

Change-Id: Ib2fa2d80b3046909ed2763545410d5d95f464b4d
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>