Added planned / timed slides to owlcarousel

[thexmanxyz]

@newkind @mahagr
This PR can be seen as an addition to a previous merged PR (#2286) where I added the disable functionality for owlcarousel slides. I now had the time to create a more sophisticated solution for the Particle which I already had in mind for months.

It is fine to have slides that can be disabled on demand but even better for maintenance of a websites would be to have specific time intervals to show slides. However, that still wasn't enough, it should be possible to define recurring events (e.g. new year, x-mas, holidays, whatever) and not only within a specific one-time frame.

Therefore I extended the previous behaviour so it is still possible to enable / disable items like before. Additionally, I added two improved options which allow us to show slides on a one-time or an annually event basis. To sum things up we have now the following options to show slides:

• yes: show always
• no: show never
• annual: show every year when the current date is in the time frame
• once: show only when the current date is in the exact time frame

PS.: all edge cases should be covered like one-day events, empty start or end-dates as well as year leaps

[thexmanxyz]

@mahagr I probably need to remove caching here to allow the feature to work correctly but I'm not sure. Provide me some input and I will add the respective commit removing it.

[mahagr]

Just FYI on caching, the static caching is meant to use on a content that is known never to change (unless the particle gets saved).

[thexmanxyz]

@mahagr Alright, so we need to remove caching here to avoid issues (make it even work). In the meantime I also tested that with some events and can also confirm that. I will prepare a commit in the next minutes. Thanks for the additional feedback.