Flush every pending settings write to the backend and block until it lands.
GSettings writes are delivered asynchronously, so a short-lived process (e.g. gpaste-client) must call this before it acts on the change or exits — without it a write can still be in flight, and another process reading the key (or the same process re-executing) would see the old value. Flushes the whole process' GSettings traffic, not just this instance's.
| this |
a Settings instance |