When using 'Send a file/URL' in the indicator with multiple devices, the
device the user selects for file sharing is not automatically selected in
the URL handler dialog that follows.
This happens because `model.rowForDevice(deviceId)` is called without
waiting for the model to be populated, always returning -1.
Resolving this exposed another bug, where the selected device was still
not the one picked. This happens because `proxyModel` is used to fill the
`QComboBox` but `model` is used to find the correct index. Since
`DevicesPluginFilterProxyModel` sorts and filters the data from `model`
the indices do not match.
This commit addresses these issues by deferring the device selection until
data is available and implementing `rowForDevice` for filtered/sorted data
in `DevicesPluginFilterProxyModel`.
