add bDisallowPrivate parameter to LanguageTag::isValidBcp47()

Change-Id: I0924b0fa268c38c238226d082cf26595bba72f0c
(cherry picked from commit fecde4c7099fdb7c5ee396b0f68ec517f3a08955)
add unit test for LanguageTag::isValidBcp47() with bDisallowPrivate

Change-Id: I58089bc00c640bb1d8461d8f482ad623f9566e76
(cherry picked from commit a31f53d0587b39bec9dca62b49a63cee0f08f3fe)
and some replacement code for lt_tag_get_privateuse(), sigh..

Change-Id: Ida5064f32a16269b16ec16c0ce9f42ea895641f9
(cherry picked from commit 4516b18f9a2e9c1bbbf013ccaedf0a526ba69dea)
disallow private use language tags in combo box edit

Change-Id: I75f0b93507d31a12b07e4bb7d2c86671aa074486
(cherry picked from commit a8c5adabb98d4152527b553f904c1841c7393748)