IrcDecoder: fall back to the first available plugin
IrcDecoder: fall back to the first available plugin

Instead of hardcoding "uchardet". The original plan was to make
"uchardet" always available, but now the build system makes it
possible to exclude it from build.

communi/communi-symbian,communi/communi-desktop,communi/communi-symbian,communi/communi-symbian,communi/communi-symbian,communi/communi-desktop,sevanteri/communi-desktop,gdamjan/communi-desktop,jpnurmi/communi-desktop