build: consider all translation keywords for C++

And document which aprts are still missing.
The keyword list is are copied from make_pot.sh.
Sorting by file helps keeping future PO(T) update diffs small'ish
and is also done in wangqr’s PO-file update procedure. (POT file
generation also sorts, but by preapplying `LC_ALL=C sort` to the
file list instead of using xgettext’s sorting option.)

Without this, meson’s aegisub-pot missed the majority of translatable
strings from C++ sources. While this brings the target much closerto
make_pot.sh, the remaining strings aren't straigthforward to integrate
unfortunately. At the time of writing meson has no support for iss files
and make_pot.sh uses custom logic to select only specific strings
from Lua and JSON files.

Even after this commit, POT updates should
thus still continue to use make_pot.sh.

po/make_pot.sh

@@ -13,7 +13,8 @@ maybe_append() {
 }
 
 find ../src ../src/command -name \*.cpp -o -name \*.h \
-  | xgettext --files-from=- -o - --c++ -k_ -kSTR_MENU -kSTR_DISP -kSTR_HELP -kfmt_tl -kfmt_plural:2,3 -kwxT -kwxPLURAL:1,2 \
+  | xgettext --files-from=- -o - --c++ --sort-by-file \
+             -k_ -kSTR_MENU -kSTR_DISP -kSTR_HELP -kfmt_tl -kfmt_plural:2,3 -kwxT -kwxPLURAL:1,2 \
   | sed 's/SOME DESCRIPTIVE TITLE./Aegisub 3.2/' \
   | sed 's/YEAR/2005-2014/' \
   | sed "s/THE PACKAGE'S COPYRIGHT HOLDER/Rodrigo Braz Monteiro, Niels Martin Hansen, Thomas Goyne et. al./" \

po/meson.build

@@ -1,4 +1,14 @@
 i18n = import('i18n')
 # This is currently busted on OSX
+# and incomplete on any platform.
+# It misses translatable strings not directly found in either
+# C++ source, desktop or appdata file. This affects strings
+# of the Windows installer (iss), from Lua scripts and JSON files.
+# Until a solution is found, POT updates should continue to use make_pot.sh.
 i18n.gettext('aegisub',
+	         args: [
+	            '-k_', '-kSTR_MENU', '-kSTR_DISP', '-kSTR_HELP', '-kwxT',
+	            '-kfmt_tl', '-kfmt_plural:2,3', '-kwxPLURAL:1,2',
+	            '--sort-by-file'
+	         ],
 	         install_dir: localedir)