Switch auto-capture from onCreated to onDeterminingFilename, which
fires once Chrome has already resolved the real filename (from
Content-Disposition or the URL). That name is now passed through to
the native host and used to build a full dir+filename destination for
addTransfer, replacing showNewTransferDialog. A non-empty destDir
makes KGet skip its interactive dialog and start immediately, which
also fixes it showing an opaque URL path segment as the filename for
signed/obfuscated URLs (e.g. cloud storage links) instead of the real
name.
Also enable KGet's existing (but disabled-by-default) "Download
Finished" KNotify popup via install.sh, so there's still feedback now
that captures no longer show an interactive dialog.