Disable key snooping by default.

Key snooping has been deprecated since gtk+-3.4.
Refector the key-press handling.
Handle special keys correctly.
Send preedit-changed event when composing preedit string.
Replace INCLUDES with AM_CPPFLAGS
Fix bootstrap
Get rid of deprecated gtk symbols
Fallback to gtk_draw_insertion_cursor
Merge branch 'develop'
Bump release 1.4.15