Summary:
It serves as a replacement for `kdbusaddons_generate_dbus_service_file`.

An application can be a DBus-activated service just fine without using KDBusAddons.
Moreover, this new module uses named arguments for future-proofing, and adds support for specifying a `SystemdService`.
It also cleans up the confusion on what the "path" is about: Rather than requiring to specify executable and path separately, we just extract the executable file name on Windows, if necessary.

Usage:
```
ecm_generate_dbus_service_file(NAME org.kde.kded5
                               EXECUTABLE ${KDE_INSTALL_FULL_BINDIR}/kded5
                               SYSTEMD_SERVICE plasma-kded)
```

Test Plan:
* Was able to generate a kded service file
* Was able to generate a kded service file with `SystemdUnit`
* Verified that it moaned when executable wasn't an absolute path
* Untested on Windwos

Reviewers: #frameworks, davidedmundson, kossebau, kfunk, habacker

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D29051