Translations: Use POSIX-style locale identifiers

Switch to using an underscore to separate ISO-639-1 language and
ISO-3166-1 country codes (instead of a dash).

Also removes the `lang_map` setting from the Transifex client
configuration that replaces the underscore with a dash:
https://docs.transifex.com/client/client-configuration#language-mappings

This fixes the broken loading of translations that used a dash in the
filename, because this is not supported by `QTranslator::load()`.
See https://doc.qt.io/qt-5/qtranslator.html#load-1 for details.

.tx/config

@@ -1,6 +1,5 @@
 [main]
 host = https://www.transifex.com
-lang_map = af_ZA: af-ZA, am_ET: am-ET, ar_AA: ar-AA, ar_AE: ar-AE, ar_BH: ar-BH, ar_DZ: ar-DZ, ar_EG: ar-EG, ar_IQ: ar-IQ, ar_JO: ar-JO, ar_KW: ar-KW, ar_LB: ar-LB, ar_LY: ar-LY, ar_MA: ar-MA, ar_OM: ar-OM, ar_QA: ar-QA, ar_SA: ar-SA, ar_SY: sy-IQ, ar_TN: ar-TN, ar_YE: ar-YE, arn_CL: arn-CL, as_IN: as-IN, ast_ES: ast-ES, az_AZ: az-AZ, ba_RU: ba-RU, be_BY: be-BY, bg_BG: bg-BG, bn_BD: bn-BD, bn_IN: bn-IN, bo_CN: bo-CN, br_FR: br-FR, bs_BA: bs-BA, ca_ES: ca-ES, co_FR: co-FR, cs_CZ: cs-CZ, cy_GB: cy-GB, da_DK: da-DK, de_AT: de-AT, de_CH: de-CH, de_DE: de-DE, de_LI: de-LI, de_LU: de-LU, dsb_DE: dsb-DE, dv_MV: dv-MV, el_GR: el-GR, en_AU: en-AU, en_BZ: en-BZ, en_CA: en-CA, en_GB: en-GB, en_IE: en-IE, en_IN: en-IN, en_JM: en-JM, en_MY: en-MY, en_NZ: en-NZ, en_PH: en-PH, en_SG: en-SG, en_TT: en-TT, en_US: en-US, en_ZA: en-ZA, en_ZW: en-ZW, eo: eo-XX, es_AR: es-AR, es_BO: es-BO, es_CL: es-CL, es_CO: es-CO, es_CR: es-CR, es_DO: es-DO, es_EC: es-EC, es_ES: es-ES, es_GT: es-GT, es_HN: es-HN, es_MX: es-MX, es_NI: es-NI, es_PA: es-PA, es_PE: es-PE, es_PR: es-PR, es_PY: es-PY, es_SV: es-SV, es_US: es-US, es_UY: es-UY, es_VE: es-VE, et_EE: et-EE, eu_ES: eu-ES, fa_IR: fa-IR, fi_FI: fi-FI, fil: fil-PH, fo_FO: fo-FO, fr_BE: fr-BE, fr_CA: fr-CA, fr_CH: fr-CH, fr_FR: fr-FR, fr_LU: fr-LU, fr_MC: fr-MC, fy_NL: fy-NL, ga_IE: ga-IE, gd: gd-GB, gl_ES: gl-ES, gu_IN: gu-IN, ha: ha-NG, he_IL: he-IL, hi_IN: hi-IN, hr_HR: hr-HR, hsb: hsb-DE, ht_HT: ht-HT, hu_HU: hu-HU, hy_AM: hy-AM, id_ID: id-ID, ig_NG: ig-NG, is_IS: is-IS, it_CH: it-CH, it_IT: it-IT, ja_JP: ja-JP, ka_GE: ka-GE, kk_KZ: kk-KZ, kl: kl-GL, km_KH: km-KH, kn_IN: kn-IN, ko_KR: ko-KR, ku_IQ: ckb-IQ, ky: ky-KG, lb: lb-LU, lo_LA: lo-LA, lt_LT: lt-LT, lv_LV: lv-LV, mi: mi-NZ, mk_MK: mk-MK, ml_IN: ml-IN, mn_MN: mn-MN, mr_IN: mr-IN, ms_MY: ms-MY, mt_MT: mt-MT, my_MM : my-MM, nb_NO: nb-NO, ne_NP: ne-NP, nl_BE: nl-BE, nl_NL: nl-NL, nn_NO: nn-NO, nso: nso-ZA, oc: oc-FR, or_IN: or-IN, pa_IN: pa-IN, pl_PL: pl-PL, ps: ps-AF, pt_BR: pt-BR, pt_PT: pt-PT, ro_RO: ro-RO, ru_RU: ru-RU, rw: rw-RW, sah: sah-RU, sd: sd-PK, se: se-FI, se_NO: se-NO, se_SE: se-SE, si_LK: si-LK, sk_SK: sk-SK, sl_SI: sl-SI, sma: sma-SE, sq_AL: sq-AL, sr@latin: sr-YU, sr_BA: sr-BA, sr_CS: sr-CS, sr_ME: sr-ME, sr_RS: sr-RS, sv_FI: sv-FI, sv_SE: sv-SE, sw_KE: sw-KE, ta_IN: ta-IN, ta_LK: ta-LK, te_IN: te-IN, tet: tet-TL, tg_TJ: tg-TJ, th_TH: th-TH, tk_TM: tk-TM, tl_PH: tl-PH, tn: tn-ZA, tr_TR: tr-TR, tt: tt-RU, tzm: tzm-DZ, ug: ug-CN, uk_UA: uk-UA, ur_PK: ur-PK, uz@Latn: uz-UZ, uz@Cyrl: uzb-UZ, vi_VN: vi-VN, wo_SN: wo-SN, xh: xh-ZA, yo: yo-NG, zh_CN: zh-CN, zh_HK: zh-HK, zh_MO: zh-MO, zh_SG: zh-SG, zh_TW: zh-TW, zu_ZA: zu-ZA
 
 [mixxxdj.mixxx2-4]
 file_filter = res/translations/mixxx_<lang>.ts