Add changelog2appdata into cmake build

Telegram/cmake/generate_appdata_changelog.cmake

@@ -0,0 +1,24 @@
+# This file is part of Telegram Desktop,
+# the official desktop application for the Telegram messaging service.
+#
+# For license and copyright information please follow this link:
+# https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+
+function(generate_appdata_changelog target_name changelog_path appdata_path)
+    find_package(Python3 REQUIRED)
+
+    add_custom_target(${target_name}_appdata_changelog
+        ${Python3_EXECUTABLE}
+        ${submodules_loc}/build/changelog2appdata.py
+        -c "${changelog_path}"
+        -a "${appdata_path}"
+        -n 10
+    COMMENT "Generating AppData changelog (${target_name})"
+    DEPENDS
+        ${submodules_loc}/build/changelog2appdata.py
+        ${changelog_path}
+        ${appdata_path}
+    VERBATIM
+    )
+    add_dependencies(${target_name} ${target_name}_appdata_changelog)
+endfunction()