This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

This makes for cleaner imports.

Since the array of 'ClozeElements' can be empty, it must be checked

The factory was ignoring the src parameter, which is set before by the 
UnitService.

Instead, use :host in conjunction with ::deep

- Unify font and surface props
- Use intersection type for additional properties
- Also rework editor props panel to properly create the combined 
properties recursively, passing the results to its child panel (tabs).
New algorithm works without lodash, therefore the package is removed.

Move required styles and classes to the overlays of player and editor

- Use fixedSize always together with dynamicPositioning!
- Rename css class 'center-content' to 'fixed-size-element'
'fixed-size-element' ensures that the content does not overflow.

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

Remove the wrapper divs in each element component and add the
'fixed-size-content-wrapper' class to the element overlays
of player and editor

The previous solution, which uses a top position of 50%, did not work on
iPad

The former outline was always displayed square on the iPad

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

The package was not a proper ECMA Script package and therefore can not 
be tree shaken properly and would unnecessarily increase the package 
size.

- Simplified the code logic. Verification is done once in the beginning 
and not on every element.
- Now uses semver to verify and compare version strings

Better solution without an extra variable.

ImportModuleVersion stores the unitDefinitionType of the loaded unit
definition. During the initialization of the unit, the individual
elements can react to the stored value.

and DropListSimpleElement

Use singleton instance of IdService in UIElement

This is not a complete backwards compatibility. Only the value "0" can
be safely classified as false. It is therefore set to "1".