ui/gtk3: Support Shift-Space to insert a Space on Emojier preedit
ui/gtk3: Support Shift-Space to insert a Space on Emojier preedit

Implemented Shift-Space on preedit since Shift-Space had worked on
Emojier's GtkEntry in the previous release.

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