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.
Fix incorrect ETA calculation
Cleaup includes
Use Qt::GlobalColor to refer to color instead of string

Qt uses binary search to convert string to QColor, we don't need that
binary search at all. This patch could be considered as optimization, but
in reality creating QColor takes only 0.2% of time. So it should be visible
at all.

This could be considered as cleanup for not calling expensive functions
from non-expensive ones.
Remove mutable fields from TorrentModelItem

The querying of TR_STATUS doesn't affect color and icon now.
Call TorrentModelItem::state() directly in getTorrentStatusReport

There is no point in wraping/unwraping QVariant.
Split src/transferlistdelegate into .h and .cpp
Split src/transferlistfilterswidget into .h and .cpp
Split src/torrentpersistentdata into .h and .cpp
Remove default argument from TorrentPersistentData::setAddedDate()
Put directories first when sorting torrent content by name

Closes #1594.
Remove unused code

Leftovers of 4ec176b683c6 ("Make possible to move file to .unwanted
directory after downloading")
Use https for Piratebay search plugin. Closes #2093
Fix a memleak in torrentmodel.

Perhaps we should finally move to C++11 and std::unique_ptr?
Unused include
Merge pull request #2051 from pmzqla/filter-input

Improve input box to filter torrent list
Merge pull request #2059 from sorokin/fix-eta

Fix ETA calculation
Merge pull request #2095 from DoumanAsh/piratebay_https

Return https connection for piratebay