Slot View Helper (#1454)

* Slot helper prototype.

- Slot/block system for easier customization.
- Designed to set block values then include parent template (helper coming).
- start/end for buffer capture.
- set for string setting.
- clear for value clearing.

* Add get method to Slot.

* Add Slot method documentation.

* Improve Slot documentation.

* Change syntax to slot('name')->action(). Fix instancing.

* Improve documentation.

* Add examples to slot PR.

* Update documentation.

* checkstyles

* Fix error with slot->get.

* Remove trim so any value type can be saved.

* Add prepend/append methods to slots.

* Move Slots helper to VuFindTheme.

* Add unit tests for Slot helper.

* Clear Slot prepends and appends.

* Space and trim block output.

* checkstyle typo

* Improve documentation of non-string slot values.

* Add method constants.

* Add __invoke(name, val) and __toString shortcuts.

* Clarify prepend/append concatination.

* Slot clear returns old contents.

* Update test to match new clea...