Allow using space as next page key even if "space as selection" option is not enabled

[kanru]

Is your feature request related to a problem? Please describe.

ibus-chewing has a workaround that always issue PageDown when handling the space key in selection mode.

    /* Bug of libchewing:
     * If "Space as selection" is not enabled, Space key cannot be used
     * to turn pages or switch the length of candidate lis.
     */
    if (table_is_showing) {
        return self_handle_page_down(self, kSym, unmaskedMod);
    }

This misses out some special handling of the Space key like narrowing the candidate length.

Describe the solution you'd like

Always allow space key as select key when in selection mode.