Mechanism to register & launch apps in HMI

We need a mechanism for UI apps to register themselves to HMI so HMI shows them in the menus and knows how to launch them. We'll need at least the following information from apps:

• Name (preferably localized)
• Icon (Apparently in SVG format for some reason)
• Path to binary

We already have a file format for this so I'd recommend we just use that: https://standards.freedesktop.org/desktop-entry-spec/latest/ . glib has API to parse them even so it makes lives much easier.

Previously, the idea was for lifecycle components to directly launch apps (at least in certain cases) through service files but we ditched that idea for a bunch of reasons but mainly cause service files don't make sense for UI apps and UI apps are not really services. So idea now is that HMI will be managing apps mostly and apps need not know too much about lifecycle (except for saving state on shutdown and recovering from last saved state).

Steps:
1. Create a sample desktop file for one of the existing HMI apps for testing purposes.
2. Add support for lauching desktop files from the HMI.
3. Remove support for launching service files from the HMI.