Add /avatar set command to publish avatar

Use `/avatar set <path>` where <path> is an image file to upload a new
avatar for the current user. When the avatar is too big it gets scaled
down. Scaling code copied from dino.

Fixes https://github.com/profanity-im/profanity/issues/1687

src/xmpp/avatar.h

@@ -40,5 +40,6 @@
 
 void avatar_pep_subscribe(void);
 gboolean avatar_get_by_nick(const char* nick, gboolean open);
+gboolean avatar_set(const char* path);
 
 #endif