core: image convertion + SVG support:

/!\ new optional dependency: CairoSVG (with installed `[SVG]` extra)

- new `convert` method in `tools.image` to save an image in an other format, with support
  for SVG (when CairoSVG is available)
- new `imageConvert` method is available for frontends

setup.py

@@ -53,6 +53,10 @@ install_requires = [
     'omemo-backend-signal',
 ]
 
+extras_require = {
+    "SVG": ["CairoSVG"],
+}
+
 DBUS_DIR = 'dbus-1/services'
 DBUS_FILE = 'misc/org.salutatoi.SAT.service'
 with open(os.path.join(NAME, 'VERSION')) as f: