client/gtk2: Keep preedit cursor_pos and visible in clearing preedit text
client/gtk2: Keep preedit cursor_pos and visible in clearing preedit text

Clear the preedit_string but keep the preedit_cursor_pos and
preedit_visible because a time lag could happen, firefox commit
the preedit text before the preedit text is cleared and it cause
a double commits of the Hangul preedit in firefox if the preedit
would be located on the URL bar and click on anywhere of firefox
out of the URL bar.

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