meson: Ensure clapper uses local translations within the devenv

This patch modifies the initialization routines of clapper to check for
    CLAPPER_GTK_OVERRIDE_LOCALEDIR and
    CLAPPER_APP_OVERRIDE_LOCALEDIR
and uses that instead of the LOCALEDIR specified in the config.h.

It also fixes the bug where libclapper-gtk loads the translations for
the clapper application and the clapper application loads the
translations for the libclapper-gtk. (It took me shockingly long to
figure out why the translations weren't working 🙃)

Co-authored-by: Rafał Dzięgiel <rafostar.github@gmail.com>
Signed-off-by: Florian "sp1rit"​ <sp1rit@disroot.org>

meson.build

@@ -19,6 +19,8 @@ clapper_version_suffix = '-' + version_array[0] + '.0'
 
 clapper_api_name = meson.project_name() + clapper_version_suffix
 
+devenv = environment()
+
 gnome = import('gnome')
 pkgconfig = import('pkgconfig')
 i18n = import('i18n')
@@ -124,6 +126,8 @@ endif
 subdir('src')
 subdir('doc')
 
+meson.add_devenv(devenv)
+
 summary({
   'prefix': prefix,
   'bindir': bindir,