GPencil: Add new parameter to set caps in Cutter

The new parameter allows to define if after cutting the stroke the cap of the cut side will be set as flat. 

Before, the cap shape of the cut side always was equal to the original stroke, and in some situations, the rounded cap was visible.

Note: If the angle of the join is very extreme,  it's still possible to view some sections of the cut stroke.,

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1814,6 +1814,11 @@ class _defs_gpencil_paint:
 
     @ToolDef.from_fn
     def cutter():
+        def draw_settings(context, layout, tool):
+            props = tool.operator_properties("gpencil.stroke_cutter")
+            row = layout.row()
+            row.use_property_split = False
+            row.prop(props, "flat_caps")
         return dict(
             idname="builtin.cutter",
             label="Cutter",
@@ -1821,6 +1826,7 @@ class _defs_gpencil_paint:
             cursor='KNIFE',
             widget=None,
             keymap=(),
+            draw_settings=draw_settings,
         )
 
     @ToolDef.from_fn