build: Factor out qt translations from build system

Move qt translations to a separate make include file.
This makes it easier to auto-generate this list from tooling
(see bitcoin-core/bitcoin-maintainer-tools#36).

doc/translation_process.md

@@ -73,7 +73,7 @@ To assist in updating translations, a helper script is available in the [maintai
 ```bash
 git ls-files src/qt/locale/*ts|xargs -n1 basename|sed 's/\(bitcoin_\(.*\)\).ts/        <file alias="\2">locale\/\1.qm<\/file>/'
 ```
-4. Update `src/Makefile.qt.include` manually or via
+4. Update `src/Makefile.qt_locale.include` manually or via
 ```bash
 git ls-files src/qt/locale/*ts|xargs -n1 basename|sed 's/\(bitcoin_\(.*\)\).ts/  qt\/locale\/\1.ts \\/'
 ```