Restore settings label and prevent warning when Baloo is unavailable

Summary:
As discussed in D14814, 6674c9c3 removed the {nav Show tooltips}
checkbox in Dolphin's preferences when Baloo is unavailabe at compile
time and thus the tooltip would not be functional. The {nav Miscellaneous}
label in front of the checkbox was also removed, even though it relates
to all options in that group.

In addition, after 6674c9c3 running Dolphin without Baloo support
would print
  QWidget::insertAction: Attempt to insert null action
on startup.

Both issues can be fixed with more `#ifdefs`.

Test Plan:
After building with `-DCMAKE_DISABLE_FIND_PACKAGE_KF5Baloo=ON`, the
{nav Show tooltips} checkbox is removed, the {nav Miscellaneous} label
is still there, and there is no warning printed.

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D14980