Dolphin Service Installer: Run uninstall scripts, rename methods

Summary:
Now scripts starting with `uninstall` are being executed.
Addtionally the `runInstallerScriptOnce` method has been renamed, because it is used to run the install/uninstall scripts.
Same for `runInstallerScript`.
Depends on D28801

Test Plan: Compile, run tests. Try to uninstall a dolphin service which has a uninstall.sh script.

Reviewers: #dolphin, ngraham, nicolasfella, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: cfeck, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D28836