Dynamic default command lookup

The default command was being determined once (when plugin settings were being loaded). This patch lets the plugin make the lookup every time the user requests to run the plugin's "associated command".

Updated list of default commands (ordered by priority):

1. xfce4-taskmanager
2. htop (new with this patch)
3. top

Making the lookup dynamic covers the following use case:

• If the user installs or uninstalls xfce4-taskmanager then the cpugraph plugin does not need to be restarted and does not need to reconfigured by the user (assuming that the "Associated command" field is already empty)

Lastly, if the "Associated command" field in the configuration dialog is empty, then the checkboxes below this field are now disabled in order to signal to the user that they have no effect.

Closes: #5 (closed)