Qt: fix a crash when closing a file while capture file properties window is open

refresh_button_ and copy_comment_button_ are not used neither initialized, get rid of them
Also clear text areas and disable buttons

Change-Id: I418d712313041dcf4f15aceee01497f01ae74918
Reviewed-on: https://code.wireshark.org/review/5927
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>