Add customfield Filter to courselist shortcode

[georgmaisser]

copy function from urise project. the goal ist to be able to add any filter on a customfield to the place where we use the shortcode

• Fix standard custom field filter to be shown in standard filters menu - needs to work with "select" customfield (Auswahlliste)
• Make sure actual values of customfield are shown, currently indexes are shown (1, 2,...)
• Implement or copy pre-filter not to be shown in filter menu but to filter BEFORE booking options are shown
• example: argument tag=Highlighted should only show booking options with a booking option customfield with shortname tag that has the value "Highlighted"
• this should also work with for example category="Kommunikation" (see Sandra's screenshot)
• implement filterontop

[bernhard-wunderbyte]

@plomersan

[plomersan]

[bernhard-wunderbyte]

@plomersan Important: When using shortcode for select (Auswahlliste) we cannot use the actual string value, but instead have to use the index.

Example: For a customfield with shortname "tag" [courselist cmid=12 all=true tag=Highlighted] won't work, but we can use [courselist cmid=12 all=true tag=1] if "Highlighted" is the value of the first row.

[bernhard-wunderbyte]

[courselist cmid=12 all=true filter=1 filterontop=1] will show the filter on top now.