Rename: groupchats -> conferences

This is in line with the toxcore API naming scheme and is in preparation
for the merge with the new groupchat implementation

src/execute.h

@@ -31,7 +31,7 @@
 enum {
     GLOBAL_COMMAND_MODE,
     CHAT_COMMAND_MODE,
-    GROUPCHAT_COMMAND_MODE,
+    CONFERENCE_COMMAND_MODE,
 };
 
 void execute(WINDOW *w, ToxWindow *self, Tox *m, const char *input, int mode);