fix plugin restart prompt removing the plugin when disregarding the dialog

Moved from https://github.com/xfce-mirror/xfce4-panel/pull/3 as requested.

Previously when prompted to either restart or permanently remove a plugin from the panel, hitting escape (ESC) on the keyboard to close the sudden popup dialog would choose the destructive action. This patch changes it to the harmless one: restarting the plugin.

The current behavior has caught me off guard several times, causing me to nuke the xfce4-docklike-plugin with its configuration.

Full disclosure: I did not build or test this patch. My system is currently too old to do so. I hope to have this change after upgrading in the not too distant future though. 😉
Would you be so kind and test it for me, or maybe its even a simple enough change to not need any testing?