Gitignored test build products.
Made aggregate project for tests and enabled `make check`.
Remove unnecessary cleanup from test.

VideoTableModel's dtor already does that.
Fixed memory leak in a test case.
Fix VideoTableModel::removeRows bounds checks.
Removed null checks after `new`.

`new` should throw `std::bad_alloc` instead of returning null.
Fixed operator precedence mistake in a test.
Fix compiler warnings. Mostly unused variables.
Cleanup of MainWindow.cpp.
Moved yle-dl extra options field to detailsWidget.

It's an advanced option that'd be just UI clutter for
the vast majority of users.
Disable download button while the list is empty.