Properly disable create_dir action in Trash

`isFolderWritable` is set in DolphinView::updateWritableState() according to
KFileItemListProperties::supportsWriting().
However, `writing=true` does not imply `makedir=true`,
in an ioslave's .protocol file. An example is the trash protocol.

So we need to enable the `create_dir` action only if the protocol is
actually able to create folders.

CCBUG: 332463
REVIEW: 129712