build: Use XLIFF file to provide more context to Transifex translators

Details: https://docs.transifex.com/formats/xliff

.tx/config

@@ -3,5 +3,5 @@ host = https://www.transifex.com
 
 [bitcoin.qt-translation-021x]
 file_filter = src/qt/locale/bitcoin_<lang>.ts
-source_file = src/qt/locale/bitcoin_en.ts
+source_file = src/qt/locale/bitcoin_en.xlf
 source_lang = en

build-aux/m4/bitcoin_qt.m4

@@ -225,6 +225,7 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
   BITCOIN_QT_PATH_PROGS([RCC], [rcc-qt5 rcc5 rcc], $qt_bin_path)
   BITCOIN_QT_PATH_PROGS([LRELEASE], [lrelease-qt5 lrelease5 lrelease], $qt_bin_path)
   BITCOIN_QT_PATH_PROGS([LUPDATE], [lupdate-qt5 lupdate5 lupdate],$qt_bin_path, yes)
+  BITCOIN_QT_PATH_PROGS([LCONVERT], [lconvert-qt5 lconvert5 lconvert], $qt_bin_path, yes)
 
   MOC_DEFS='-DHAVE_CONFIG_H -I$(srcdir)'
   case $host in
@@ -258,7 +259,10 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
       AC_MSG_ERROR([libQtDBus not found. Install libQtDBus or remove --with-qtdbus.])
     fi
     if test "x$LUPDATE" = x; then
-      AC_MSG_WARN([lupdate is required to update qt translations])
+      AC_MSG_WARN([lupdate tool is required to update Qt translations.])
+    fi
+    if test "x$LCONVERT" = x; then
+      AC_MSG_WARN([lconvert tool is required to update Qt translations.])
     fi
   ],[
     bitcoin_enable_qt=no

src/Makefile.qt.include

@@ -356,6 +356,8 @@ $(srcdir)/qt/bitcoinstrings.cpp: FORCE
 translate: $(srcdir)/qt/bitcoinstrings.cpp $(QT_FORMS_UI) $(QT_FORMS_UI) $(BITCOIN_QT_BASE_CPP) qt/bitcoin.cpp $(BITCOIN_QT_WINDOWS_CPP) $(BITCOIN_QT_WALLET_CPP) $(BITCOIN_QT_H) $(BITCOIN_MM)
 	@test -n $(LUPDATE) || echo "lupdate is required for updating translations"
 	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(LUPDATE) $^ -locations relative -no-obsolete -ts $(srcdir)/qt/locale/bitcoin_en.ts
+	@test -n $(LCONVERT) || echo "lconvert is required for updating translations"
+	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(LCONVERT) -o $(srcdir)/qt/locale/bitcoin_en.xlf -i $(srcdir)/qt/locale/bitcoin_en.ts
 
 $(QT_QRC_LOCALE_CPP): $(QT_QRC_LOCALE) $(QT_QM)
 	@test -f $(RCC)