i18n: drop intltool use, use meson's merge_file directly

This didn't work during the initial conversion to meson, but should now.
A sufficiently new polkit is also required, for the .its rules files.

Note that https://github.com/mesonbuild/meson/blob/master/docs/markdown/i18n-module.md
says that 'install' argument was added in meson 0.43.0. If this is accurate,
warnigs might be generated with older mesons. Fedora has 0.43.0 across the
board, but other distros probably don't, but I guess that a warning is
prefereable to having to update do latest meson.

The advantages are:
- one less dependency (intltool)
- using the generic implementation instead of our open-coded calls
- we don't need to use the fake "_" prefixes in XML

Replaces #1609, fixes #7300.