Add render mode to text label

Add render mode to text label

Sync : default, synchronous text loading.
AsyncAuto : automatically requests an asynchronous text load in OnRelayout.
AsyncManual : users should manually request rendering using the async text method.

All text rendering processes (update/layout/render) are performed asynchronously in AsyncAuto and AsyncManual.

Removed unnecessary properties.

Change-Id: Ie33be323f900cc8c5a9052448c236b1f4e921965
Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>