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

sat/bridge/bridge_constructor/bridge_template.ini

@@ -1009,3 +1009,18 @@ doc=Generate a preview of an image in cache
 doc_param_0=image_path: path of the original image
 doc_param_1=%(doc_profile_key)s
 doc_return=path to the preview in cache
+
+[imageConvert]
+async=
+type=method
+category=core
+sig_in=ssss
+sig_out=s
+doc=Convert an image to an other format
+doc_param_0=source: path of the image to convert
+doc_param_1=dest: path to the location where the new image must be stored.
+    Empty string to generate a file in cache, unique to the source
+doc_param_3=extra: serialised extra
+doc_param_4=profile_key: either profile_key or empty string to use common cache
+    this parameter is used only when dest is empty
+doc_return=path to the new converted image