This adds support for custom gutter template customization.
There are three main features included:

1. A new structural directive to allow gutter template customization (asSplitGutter).
   The usage as follows:

   <as-split>
     <div *asSplitGutter>...</div>
     <as-split-area>...</as-split-area>
     ...
   </as-split>

   The new asSplitGutter directive allows access to the following context:

   export interface SplitGutterTemplateContext {
     /**
     * The area before the gutter.
     * In RTL the right area and in LTR the left area
     */
     areaBefore: IArea
     /**
     * The area after the gutter.
     * In RTL the left area and in LTR the right area
     */
     areaAfter: IArea
     /**
     * The absolute number of the gutter based on direction (RTL and LTR).
     * First gutter is 1, second is 2, etc...
     */
     gutterNum: number
     /**
      * Whether this is the first gutter.
      * In RTL the most right area and in LTR the most left area
      */
     first: boolean
     /**
      * Whether this is the last gutter.
      * In RTL the most left area and in LTR the most right area
      */
     last: boolean
     /**
     * Whether the gutter is being dragged now
     */
     isDragged: boolean
   }

2. In case the gutter should have a drag handle allowing dragging only from it, a second directive is introduced - asSplitGutterDragHandle.
   The usage is:

   <div *asSplitGutter>
     <div asSplitGutterDragHandle>...</div>
   </div>

3. In case a certain element inside the gutter should block dragging (a button for example), a third directive is introduced - asSplitGutterExcludeFromDrag.
   The usage is:

   <div *asSplitGutter>
     <div asSplitGutterExcludeFromDrag>...</div>
   </div>

Implementation notes:

1. asSplitGutterDragHandle can be declared on multiple elements which means all of them are drag handles.
2. asSplitGutterExcludeFromDrag can be declared on multiple elements which means all of them are excluded from drag.
3. asSplitGutterExcludeFromDrag inside asSplitGutterDragHandle works as expected (all of the element is a handle except the excluded area)
4. asSplitGutterDragHandle inside asSplitGutterExcludeFromDrag is ignored and the whole element marked with exclude is excluded.
5. asSplitGutter mostly requires width: 100% and height: 100% in order to fill the whole gutter.
6. If the custom gutter is wider than the declared gutter size (using gutterSize input) the areas will overflow too. So attention is required to match the size of the gutter.

Closes #344
Closes #266
Closes #227
Closes #163
Closes #361