Flatpak C conventions

• Prefer standard C integer types over GLib typedefs when either is suitable. For example, use size_t, uint8_t, uint32_t, and int64_t instead of gsize, guint8, guint32, and gint64.
• Use size_t for array indexes, collection lengths, and loop counters compared with a size or length.
• Keep GLib types when required by a GLib API, when changing the type would alter its signedness or width, or when matching an existing public API.