Connect after on the entry 'changed' event

For some strange reason, this needs to be connected *after*. When typing an
invalid character (e.g. 'a'), the entry becomes invalid.  Then erase that
character and type it again. Now the entry is valid because get_text() returns
''. Connecting 'after' in the event seems to solve the problem.

Change-Id: I71349648a313cc1958696bb80ee4d210fc50f1c0