Show input box to filter torrent list only when the list is shown

It's misleading to show the filter on every tab when it works only
for the torrent list.
Add placeholder text to the input to filter the torrent list

Users can mistakenly think the input box is to search for torrents
and not to filter the torrent list. A placeholder text will make things
clear.
Focus transfer list on startup

The torrent list filter will be focused otherwise and its placeholder
text will be hidden on startup.
Merge pull request #2051 from pmzqla/filter-input

Improve input box to filter torrent list