UI: Avoid slight text color flashing while clicking preview tiles

Text of preview-tile widgets would change color while pressing the mouse
button on them, this wasn't useful and just made the UI feel glitchy.
The widget background already indicates the state well.

Preview-tile widgets are currently only used in the asset view template.
More usages will be introduced in the asset-shelf patch (#104831).

source/blender/editors/interface/interface_widgets.cc

@@ -4617,6 +4617,8 @@ static uiWidgetType *widget_type(uiWidgetTypeEnum type)
       wt.draw = nullptr;
       /* Drawn via the `custom` callback. */
       wt.text = nullptr;
+      /* Drawing indicates state well enough. No need to change colors further. */
+      wt.state = widget_state_nothing;
       wt.custom = widget_preview_tile;
       wt.wcol_theme = &btheme->tui.wcol_list_item;
       break;