Revert "pgui: Ignore PGEntry tab keypress"

This reverts commit 1f05d37b209470c179b6d00f888ac39ffd14bcf4.

See #994

panda/src/pgui/pgEntry.cxx

@@ -203,11 +203,6 @@ press(const MouseWatcherParameter &param, bool background) {
 
       ButtonHandle button = param.get_button();
 
-      if (button == KeyboardButton::tab()) {
-        // Tab. Ignore the entry.
-        return;
-      }
-
       if (button == MouseButton::one() ||
           button == MouseButton::two() ||
           button == MouseButton::three() ||