InputText: Internal: added reload from user-buf feature. (#2890)

Very highly requested feature (#6962, #5219, #3290, #4627, #5054, #3878, #2881, #1506, #1216, #968).
Also useful for interactive completion/selection popups (#2057, #718)
Based on @kudaba PR. Design for Inputtext V2 should make this obsolete.