Fix escape key with Ctrl+Shift+U ISO 14755.

GtkIMContextSimple uses gdk_keymap_get_for_display() to check the readable
hex codes but IBusEngineSimple ignored it because does not have the display
argument.
I added is_hex_keyval() to check the escape key and fix this bug.

BUG=http://code.google.com/p/ibus/issues/detail?id=1715
TEST=src/libibus.so

Review URL: https://codereview.appspot.com/101160044