Filters - FIX - Filter Header/Footer Duplication & Always-Visible Behaviour

[lrljoe]

Reference #1094

When you have the same filter in both header & footer, it will display on screen despite hiding the column, and will shift to another column, and also cause a duplication of that column's filter.

The issue can be seen on the demo instance by

1. Having two Columns with Secondary Header & Footer Filters (e.g. as in the Name and E-mail Columns)
2. Add anything into one of the Secondary Header/Footer filters
3. Hide that Column.

This appears to be occurring due to there being multiple matching wire:key & ID for the same filter, causing the DOM to become... upset?.

This PR introduces new methods:
setFilterPosition(string $position) // Header or Footer
hasCustomPosition(): bool // Does it Have a filter position set?
getCustomPosition(): string // Retrieves the custom position.

There's a new protected string on the Filter.php to hold the position.

The Custom Position is automatically set by the getSecondaryHeaderContents or getFooterContents functions respectively, the Filter Blades then append either header or footer in an appropriate place in the ID & wire:key respectively, blades have been updated for this.

This allows for the same Filter to exist in multiple places in the page.

There may be a better approach, but this is what I came up with!

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests and did you add any new tests needed for your feature?
2. Did you update all templates (if applicable)?
3. Did you add the relevant documentation (if applicable)?
4. Did you test locally to make sure your feature works as intended?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• [NA] Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[lrljoe]

See #1111