IO: Add FileHandler for importing SVG as curves

This enables drag-n-drop support for the `import_curve.svg` operator.
Since the existing `Import SVG as Grease Pencil` operator handles the
same file type, when dropping `.svg` files this will prompt to choose
between theses 2 operators.

See PR for examples and notes on eventually changing the operator
description.

Pull Request: https://projects.blender.org/blender/blender/pulls/140603

scripts/addons_core/io_curve_svg/__init__.py

@@ -29,7 +29,10 @@ from bpy.props import (
     StringProperty,
     CollectionProperty
 )
-from bpy_extras.io_utils import ImportHelper
+from bpy_extras.io_utils import (
+    ImportHelper,
+    poll_file_object_drop,
+)
 
 
 class ImportSVG(bpy.types.Operator, ImportHelper):
@@ -41,20 +44,25 @@ class ImportSVG(bpy.types.Operator, ImportHelper):
     filename_ext = ".svg"
     filter_glob: StringProperty(default="*.svg", options={'HIDDEN'})
 
+    directory: StringProperty(subtype='DIR_PATH', options={'SKIP_SAVE', 'HIDDEN'})
     files: CollectionProperty(
         name="File Path",
         type=bpy.types.OperatorFileListElement,
     )
 
+    def invoke(self, context, event):
+        if self.properties.is_property_set("filepath"):
+            return self.execute(context)
+        context.window_manager.fileselect_add(self)
+        return {'RUNNING_MODAL'}
 
     def execute(self, context):
         from . import import_svg
 
         if self.files:
             ret = {'CANCELLED'}
-            dirname = os.path.dirname(self.filepath)
             for file in self.files:
-                path = os.path.join(dirname, file.name)
+                path = os.path.join(self.directory, file.name)
                 if import_svg.load(self, context, filepath=path) == {'FINISHED'}:
                     ret = {'FINISHED'}
             return ret
@@ -62,20 +70,38 @@ class ImportSVG(bpy.types.Operator, ImportHelper):
             return import_svg.load(self, context, filepath=self.filepath)
 
 
+class IO_FH_svg_as_curves(bpy.types.FileHandler):
+    bl_idname = "IO_FH_svg_as_curves"
+    bl_label = "SVG as Curves"
+    bl_import_operator = "import_curve.svg"
+    bl_file_extensions = ".svg"
+
+    @classmethod
+    def poll_drop(cls, context):
+        return poll_file_object_drop(context)
+
 
 def menu_func_import(self, context):
     self.layout.operator(ImportSVG.bl_idname,
                          text="Scalable Vector Graphics (.svg)")
 
 
+classes = [
+    ImportSVG,
+    IO_FH_svg_as_curves,
+]
+
+
 def register():
-    bpy.utils.register_class(ImportSVG)
+    for cls in classes:
+        bpy.utils.register_class(cls)
 
     bpy.types.TOPBAR_MT_file_import.append(menu_func_import)
 
 
 def unregister():
-    bpy.utils.unregister_class(ImportSVG)
+    for cls in classes:
+        bpy.utils.unregister_class(cls)
 
     bpy.types.TOPBAR_MT_file_import.remove(menu_func_import)