Fixes the following error
```
qdbusxml2cpp: Got unknown type `a{sv}' processing '/build/static-compat-syncthingtray/src/syncthingtray/syncthingconnector/org.freedesktop.DBus.Properties.xml'
You should add <annotation name="org.qtproject.QtDBus.QtTypeName.Out1" value="<type>"/> to the XML description for 'changed_properties'
```
by following the suggestion of that error. Not sure why this
error occurs only now. Maybe `qdbusxml2cpp` changed in Qt 6.5.0.
* Check directly whether changed properties contain a property
instead of checking indirectly whether the returned QVariant
is invalid.
* Avoid copying the QVariant.
* Effectively this should not change anything; the code is just
more readable and possibly more efficient.
Not sure why `QString::toStdWString()` doesn't work with this compiler and
the official Qt 6.5.0 build. However, instantiating an `std::wstring`
manually works so let's just use this as a workaround.
Note that with GCC/mingw-w64 and Qt 6.4.3 the `QString::toStdWString()`
function works just fine.
Martchus