Fix zenity hacks for focusing negative answer

Originally created by @anonym on #11229 (Redmine)

Jessie’s zenity has the --default-cancel option, so we can remove the hacks we currently use to simulate it (and screw up GNOME normal order of positive vs negative answers). Affected scripts are at least:

• config/chroot_local-includes/usr/local/bin/electrum
• config/chroot_local-includes/usr/local/bin/icedove
• config/chroot_local-includes/usr/local/bin/tor-browser
• config/chroot_local-includes/usr/local/sbin/i2p-browser
• config/chroot_local-includes/usr/local/sbin/unsafe-browser

Feature Branch: feature/11229-remove-zenity-focus-hacks