tzselect: User input/output through one function

Which can be 'overriden' in the calling process as $SHOWDIALOG
With this, tzselect can be used by other scripts with a customized
user dialog. Better solutions are welcome!

Example call:
SHOWDIALOG=$PWD/tzselect_dialog tzselect

where tzselect_dialog imitates the function showdialog() of tzselect.