Fix #487: drag-selecting dynamic text fields silently converts their text source

updateForm() called on_m_text_from_cb_activated() directly, intending
only to enable the sibling widget matching the combo box's current
index ("For enable the good widget"). But that slot also loops over
every currently selected part and pushes an undo command overwriting
textFrom on any part that doesn't match, since it's normally only
reached via the combo box's own activated(int) signal (real user
interaction, never fired by programmatic setCurrentIndex()).

updateForm() runs on every selection change, so during a rubber-band
drag over dynamic text fields with different sources, each time a new
field enters the selection, the representative part's textFrom gets
force-applied to every other selected part - converting e.g. a
UserText field to ElementInfo mid-drag, before the user has released
the mouse or interacted with the combo box at all.

Split the cosmetic widget-enable logic into updateTextFromWidgetsEnabled(),
called from updateForm(). on_m_text_from_cb_activated() keeps the
part-mutating loop, now only reached from real user activation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/editor/ui/dynamictextfieldeditor.h

@@ -52,6 +52,7 @@ class DynamicTextFieldEditor : public ElementItemEditor {
 		void fillInfoComboBox();
 		void setUpConnections();
 		void disconnectConnections();
+			void updateTextFromWidgetsEnabled(int index);
 
 	private slots:
 		void on_m_x_sb_editingFinished();