Fix several bugs in generic text entry hints implementation.

Fix several bugs in generic text entry hints implementation.

The text controls contents and colour was not updated correctly in several
situations (see #12475).

The old code was completely wrong as it didn't store the actual value of the
control at all and so could never work. Do store and update it now and show
the hint if and only if the real contents is empty.

Also handle "text updated" event to correctly update the hint when the control
becomes [non-]empty.

Closes #12475.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65551 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775