Restore settings label and prevent warning when Baloo is unavailable

Summary:
As discussed in D14814, 6674c9c387d0 removed the Show tooltips
checkbox in Dolphin's preferences when Baloo is unavailabe at compile
time and thus the tooltip would not be functional. The Miscellaneous
label in front of the checkbox was also removed, even though it relates
to all options in that group.

In addition, after 6674c9c387d0 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
Show tooltips checkbox is removed, the 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