src: Show compose preedit with custom compose file
src: Show compose preedit with custom compose file

IBusEngineSimple is fixed for custom compose files
 - Show preeedit with custom compose file
 - Tentative compose preedit can be deleted by Backspace

BUG=https://github.com/ibus/ibus/issues/2058

fujiwarat/ibus,fujiwarat/ibus,ibus/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus