spice-widget: add spice_display_keyboard_ungrab()
Internally, spice-widget has separate try_keyboard_ungrab() and try_mouse_ungrab() functions to ungrab the keyboard and mouse respectively. Both of these functions are called when the grab sequence is pressed.
spice-widget exposes try_mouse_ungrab() as spice_display_mouse_ungrab() to allow callers to explicitly ungrab. However, it does not currently expose try_keyboard_ungrab(), so callers can only ungrab the mouse and cannot ungrab the keyboard.
Correct this by exposing try_keyboard_ungrab() as spice_display_keyboard_ungrab().
Fixes #73 (closed)