Palette: drag onto the tree to add inside a component; hover opens branches

A palette drag dropped on a tree row adds the component inside that row's
component, placed under its lowest child. The row under the pointer is
highlighted, and resting on a collapsed branch opens it, loading it from
the device when live without moving the selection or the open sheet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app_state.py

@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.154"
+VERSION = "0.0.0.156"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True
@@ -54,6 +54,9 @@ CLEAN_TINT = "white"
 DIRTY_NAME_COLOUR = "#c00000"
 DIRTY_NAME_TAG = "unsaved_file"
 
+# Marks the tree row a palette drag would drop into.
+TREE_DROP_TAG = "palette_drop_target"
+
 # A themed Treeview maps the selected state to SystemHighlightText, and that mapping beats
 # a row tag's own foreground -- so the file name turned white-on-blue the moment its row
 # was selected, which is exactly when the root sheet is the one being looked at. The