Simplify theme levels

[Azaziah]

At the moment OpenLP uses 3 theme levels.

Song Level = Songs, Bibles and custom slides use their own themes.
Service Level = Everything uses service theme
Global Level = Everything uses global theme

If service Level is active, changing the Global theme
has no effect and vice versa, yet they are both visible
in the UI at the same time.

Using Service/Global Level requires user to change their settings.
I think it would be much simpler to just combine the Service and
Global theme level into one and call it “Default Level” and control
it from both, service and Theme manager.

Service theme name is saved into service files,
this means the service file will change the service
theme on target system. However, this only works if
the target system has a theme with the exact same name.

I think this highly confusing and service files should
not mess around with other machines Theme settings since
every computer has their own setups/font sizes/projectors.

Alternatively, if we want to keep this, we probably should
add a setting for controlling this behauviour
(disabled by default) and try to export the theme
upon saving the the Service. (if possible)