add bDisallowPrivate parameter to LanguageTag::isValidBcp47()

Change-Id: I0924b0fa268c38c238226d082cf26595bba72f0c
add unit test for LanguageTag::isValidBcp47() with bDisallowPrivate

Change-Id: I58089bc00c640bb1d8461d8f482ad623f9566e76
and some replacement code for lt_tag_get_privateuse(), sigh..

Change-Id: Ida5064f32a16269b16ec16c0ce9f42ea895641f9
disallow private use language tags in combo box edit

Change-Id: I75f0b93507d31a12b07e4bb7d2c86671aa074486