refactor(ui5-segmented-button): rename mode to selectionMode

[tsanislavgatev]

Change the enum name from SegmentedButtonMode to SegmentedButtonSelectionMode.
The options in the enum are changed from SingleSelect and MultiSelect to Single and Multiple.
The mode property of SegmentedButton is changed to selectionMode.
Also deleted deprecated getter - selectedItem and deprecated event detail selectedItem.

BREAKING CHANGE: The mode property is changed to SelectionMode and the values it take from SingleSelect and MultiSelect to Single and Multiple. Also deleted deprecated getter - selectedItem and deprecated event detail selectedItem and now can be used with the selectedItems getter and selectedItems event detail.

Previously the application developers could set the selection mode as follows:

<ui5-segmented-button id="segButtonMulti" mode="MultiSelect">
  <ui5-segmented-button-item>Item</ui5-segmented-button-item>
  <ui5-segmented-button-item>Item</ui5-segmented-button-item>
  <ui5-segmented-button-item>Click</ui5-segmented-button-item>
  <ui5-segmented-button-item>SegmentedButtonItem</ui5-segmented-button-item>
</ui5-segmented-button>

Now the application developers could set the selection mode as follows:

<ui5-segmented-button id="segButtonMulti" selection-mode="Multiple">
  <ui5-segmented-button-item>Item</ui5-segmented-button-item>
  <ui5-segmented-button-item>Item</ui5-segmented-button-item>
  <ui5-segmented-button-item>Click</ui5-segmented-button-item>
  <ui5-segmented-button-item>SegmentedButtonItem</ui5-segmented-button-item>
</ui5-segmented-button>

Related to: #8461

[unazko]

The breaking change would be about the changing the name of the selection property to selectionMode and the type it takes.
It has to be adjusted in the PR message.

[ilhan007]

Hi @tsanislavgatev
shouldn't you also update the oficial samples, the MultiSelect sample is currently broken after this change:
https://sap.github.io/ui5-webcomponents/nightly/components/SegmentedButton/#selection-modes

[ilhan007]

The issue will be fixed in https://github.com/SAP/ui5-webcomponents/pull/8869/files