Legacy SearchEngine class really has three roles:
1. Manage search plugins,
2. Handle the search job, and
3. Handle the download of the torrent file using the search plugin.
Now it is splitted into 3 classes: SearchManager, SearchHandler and
SearchDownloadHandler.
Search GUI is also improved.
Enums are stored as strings, that improves configuration file
readability and maintainability. String values are obtained via
QMetaEnum, and since with Qt 5.5 QMetaEnum::fromType() includes a
static_assert, this has to be a safe method.
Both download handlers (in the tab and widget classes) convert model
index into an URL, set row color, and call download function.
Make the download button handler (in the SearchWidget class) call the
slot of the SearchTab class.
Since we already have searchtab.ui, let's set up all the widgets there.
Additionally, save a bit of vertical space by putting results label in
a row with the filter widgets.
