query-editor: Stop blocking expected "changed" emmission

In nautilus_query_editor_set_query(), the change_frozen flag is set which makes the entry_changed_cb() return early. This causes text to be shown in the query editor without the search actually occuring. This flag has been around "forever" and probably originally had a purpose, but now, it seems to just be preventing expected behavior.

This issue wasn't present in 42 because we were using GtkSearchEntry, which has the ::search-changed signal. The ::search-changed signal (unlike the ::changed signal for an editable) has a 150ms delay which would have bypassed the change_frozen flag.

Fixes: #2518 (closed)