New hooklist to collect avatar data from headers

Also:
 • Refactorize X-Face/Face capture as an internal plugin.
 • Add hidden preference ‘enable_avatars’ to control the internal
   capture/render process, and which allows disabling it by external
   plugins for example.
New hooklist for rendering avatars and some utils

 • Create and destroy hooklist parameter structs
 • Initialization function to install default handler
Use new internal plugin for rendering avatars

Updates and simplifies rendering logic in header pane,
message view and text view and when adding pictures to
address book.
Show header pane only when enabled
When autoselecting account for a new message, handle quoted recipient names better. Patch by Christoph Ruegge, bug #3100.
New libravatar plugin
Fix a couple of bugs

 • Incorrect initialization of radiobuttons on prefs (remember
   not to use the same variable for two purposes...)
 • Empty missing cache when changing to generated-avatar missing
   icon modes, otherwise they're still served as missing
fix overlapping labels and boxes when user has large GTK font
Fix bug #3107, " Height of row in message list does not
reflect font size - three columns, small screen layout"
Patch by Peter Sarvas.
GIT-Ignore the results of libravatar's compilation
Added Christoph Ruegge to  AUTHORS file, for bug #3100.
Fix bug #3108 “link libravatar plugin against libcurl”.

Thanks Christian Hesse for the patch.
Fix bug #3109 “libravatar: make mail address lower case before hashing MD5”

Thanks again Christian Hesse for the patch!
Improve plugin description a bit
Even more crazy idea added
A better README for libravatar plugin

 • Structure it more clearly (I hope)
 • Add a section about network issues and possible workarounds
 • Talk about plugin hidden property (yes, they're everywhere)
Add the GPG keyring as possible source for address autocompletion.
Patch by Christian Hesse, fixes bug #2868.
Fix crash due to wrong use of the gpointer source (which is a
pointer-to-pointer to GList), replace exit() with return.
fix build error: "#error GPGME was compiled with _FILE_OFFSET_BITS = 64"
tidy up and get the string translated