QmlDesigner.PropertyEditor: Fixing color conversion in LineEdit

For color properties we have to take care of the alpha channel
manually.

Change-Id: I2cbb5a2c45a16385068e771e1a1ca8a10f47a7bf
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.PropertyEditor: Removing redundant emit

We should not emit the same signal twice.

Change-Id: Iee03d0b34e91f8f32d054582fcdee18d07be89b0
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>