inputMethod: Fix delete-surrounding-text signal

Forward the arguments at the 'delete-surrounding-text' signal
from IBusInputContext to clutter_input_method_delete_surrounding()
so that ibus-typing-booster use the deleting surrounding text function.

Input method engines can delete the output text in applications
with this function.

This change will require a change of mutter of mutter!517
because the first arguemnt of the 'delete-surrounding-text' is INT
to express the offset of the current cursor position but
the first one of clutter_input_method_delete_surrounding() is UINT
since the Wayland spec accepts UINT in delete_surrounding()

mutter will change the type of the first one to INT in
clutter_input_method_delete_surrounding() to work with this change.

GNOME/mutter#539
GNOME/gnome-shell!477
4 jobs for !477 with ibus-delete-surrounding in 5 minutes and 34 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #270722
check_commit_log

00:00:56

 
  Build
passed #270723
build

00:02:56

 
  Test
passed #270724
test

00:01:37

passed #270725
test-pot

00:01:41