Fix tilemap multiline property

[lich426]

Which branch your pull-request should merge into?

• dev: Current 2.x BugFixs or Features

Describe your changes

• Multiline property issue, where obtaining values was not possible, has been resolved.
• Single-line values and multiline values in XML are distinguished from each other.

• test code

auto tilemap = TMXTiledMap::create("res/Tilemap/Test.tmx");
this->addChild(tilemap, 10, 10);

auto& properties = tilemap->getProperties();
auto it = properties.find("single_line");
if (it != properties.end())
{
    auto value = it->second;
    AXLOG("key : single_line, value : %s", value.asString().c_str());
}

it = properties.find("multi_line");
if (it != properties.end())
{
    auto value = it->second;
    AXLOG("key : multi_line, value : %s", value.asString().c_str());
}

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• I have checked readme and add important infos to this PR (if it needed).
• I have added/adapted some tests too.