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