Add app name to default notifier

[sonic2kk]

With notify-send, the default notifier, the -a flag can be used to set the name of the application. This is set to the existing $PROGNAME. It is also possible to use an --app-name= syntax, but going with -a keeps in line with the existing -i syntax for adding the icon to the notification.

This flag is not mentioned in the manual for notify-send on my Arch Linux installations, but it is mention on this page and I can verify that the flag works. Discussion and testing on other distributions may be required to make sure this doesn't raise errors or unwanted behaviours.

Example of a game launch notification with this change on KDE Plasma:

Alternatively, if this change is undesirable, steps for adding this flag manually should be included in the Notifier wiki page.

Closes #403.

[frostworx]

Thanks for the fix!
For whatever reason the -a parameter doesn't have any effect on my Arch xfce4, but that is no reason to not merge it. :)