Fix several issues with PidginAvatar in a conversation

* The `content-fit` option is only in GTK 4.8+.
* A static file can't be set as a `GdkPixbuf`, which causes warnings when
hovering over the avatar.
* The custom icon menu didn't work as it only checked for an attached buddy.
* Changing the custom icon didn't cause a refresh in the buddy icon; this is
fixed now, but in a slightly annoying way.
* Changing the custom icon in the buddy list doesn't change the icon in the
conversation window; this is not yet fixed due to missing signals.

The latter two can probably be fixed better when buddies/contacts have better
signals.

Testing Done:
Hovered over avatar in a conversation, which didn't cause any warnings.
Picked all options in Custom Icon menu from a conversation, and they all worked as expected (except for a few buddy list / conversation interactions noted above.)

Also, only tested IM conversations; probably broken in chat conversations, since there are no checks for it?

Reviewed at https://reviews.imfreedom.org/r/1689/