core: added imageCheck and imageResize methods:

imageCheck will give a report on image, notably it will tell if it's too big and needs to
be resized before a transfer.

imageResize will create a new image with the requested size and return a path to it.

sat/core/constants.py

@@ -361,6 +361,13 @@ class Const(object):
     # internationalisation
     DEFAULT_LOCALE = "en_GB"
 
+    # Contexts
+    # context indicate how things are done (e.g. to who/what a file is transfered)
+    # this is useful to make some decision (e.g. image size)
+
+    # something is done in instant messaging
+    CONTEXT_CHAT = "CONTEXT_CHAT"
+
     ## Misc ##
     SAVEFILE_DATABASE = APP_NAME_FILE + ".db"
     IQ_SET = '/iq[@type="set"]'