Admin: Styles appearing according to the available sections of the outline layout

[joomlabeat]

This PR addresses #2246

Now the Section Styles for the selected outline depend on the available sections of the outline layout.

*I have only tested this with a WP custom theme that I am building.

[mahagr]

@joomlabeat Should base outline have every style option, though?

[joomlabeat]

@mahagr - So far yes, the concept is that the base outline features all sections and styles. This is my conclusion as the best way to build templates, which in someway gantry forces us, as then all the other outlines usually inherit staff from the base, even if the have less sections.
So base outline seems to be the base of everything, which tends to feel like an acceptable convention.
I haven't tested my solution with the base not having all the sections. Do you think it would give any issues with scss?
I could test it in the coming days. Let me know if there is anything else I should take into consideration.