Refactored KLocalizedContext

Extracted everything from lib.rs
Also extracted all doc tests to integration tests.

tests/common.rs

@@ -0,0 +1,12 @@
+use lazy_static::lazy_static;
+use std::sync::Mutex;
+
+lazy_static! {
+    pub static ref TEST_MUTEX: Mutex<()> = Mutex::new(());
+}
+
+/// There can only be one thread running at the time with a QQuickEngine
+/// (in principle, everything should be in the same main thread)
+pub fn lock_for_test() -> std::sync::MutexGuard<'static, ()> {
+    TEST_MUTEX.lock().unwrap_or_else(|e| e.into_inner())
+}