Build fixes for distribution packages (GTK 4.20, system locale dir)

[switch87]

Closes #169.

Rebased on main. The schema install and the locale output dir from #169 are covered by 4cba0be / b8d63a3 (GSETTINGS_SCHEMA_DIR / GETTEXT_DIR in .cargo/config.toml), so that commit is gone. Two commits are left:

1. GTK 4.20 / libadwaita 1.8 instead of 4.22 / 1.9 (Cargo.toml). The code uses no newer API: cargo clippy --all --no-deps -- -D warnings and cargo build --release pass on current main against GTK 4.20.4 and libadwaita 1.8.7. Distributions that don't ship GTK 4.22 yet can then build it.

2. Build-time system locale dir (src/main.rs). Outside Flatpak the app binds its text domain to GETTEXT_DIR, which is also where build.rs writes the catalogs. A package builds into its work dir but installs the catalogs to /usr/share/locale, so the installed app wouldn't find them, and pointing GETTEXT_DIR at /usr/share/locale would make the build write outside its sandbox. STREMIO_LOCALEDIR=/usr/share/locale at build time (option_env!) now sets the runtime dir on its own. Flatpak still wins when FLATPAK_ID is set; without the variable GETTEXT_DIR is used as before.

A distribution build then looks like:

STREMIO_LOCALEDIR=/usr/share/locale cargo build --release
install -Dm644 target/schemas/gschemas.compiled ...   # or the .gschema.xml + glib-compile-schemas
install -Dm644 target/locale/<lang>/LC_MESSAGES/stremio.mo -t /usr/share/locale/<lang>/LC_MESSAGES/

Checked: cargo fmt --all -- --check and clippy pass; with the variable set the binary carries /usr/share/locale, without it behaves as on main.

[tymmesyde]

Hi,i updated the main branch with changes related to yours
It will now output the schemas and locales to the target folder
Those paths can be changed by setting GSETTINGS_SCHEMA_DIR and GETTEXT_DIR https://github.com/Stremio/stremio-linux-shell/blob/main/.cargo/config.toml#L4-L5

[switch87]

Thanks @tymmesyde! 4cba0be and b8d63a3 solve the schema install and the locale output dir from #169, so I dropped my schema commit and rebased on main. Two things from #169 are left in this PR: building against GTK 4.20 / libadwaita 1.8 (nothing newer is used), and STREMIO_LOCALEDIR to set the runtime locale dir separately from GETTEXT_DIR, because a package writes the catalogs to its build dir but installs them to /usr/share/locale.