Add mode to QUL Property so it manages the lifecycle

Add mode to QUL Property so it manages the lifecycle (creation, usage and destruction) of object value s as a black box. Ex, it can be used as a view for the std:string, but it does not manage the lifecycle of std:string.

Example code:

#ifndef MAIN_H
#define MAIN_H

#include <qul/object.h>
#include <qul/property.h>
#include <cstring>
#include <string>

struct MyObject : public Qul::Object {
Qul::Property<std::string> variable;

MyObject() {
variable.setValue("Hello World!");
const char *myString = variable.value().c_str();
m_myString = foo(myString);

Qul::PlatformInterface::log("string is %s: ", m_myString.data());
}

static std::string foo(const char* data) {
return data;
}

std::string m_myString;;
};

#endif // MAIN_H

This would be important for example in the use case where property is used for storing an IP address passed to the IP stack - you really don't know how long the pointer has to be alive, and when std::string is in property there is very little control over buffer. So, QML code accessing the property can cause buffer reallocation and hence a dangling pointer.