[Dashboard] Cannot delete generated widgets

[lasanthaS]

Description:
In the current implementation, once a widget is generated via the 'Widget generation wizard' it is getting stored in the underlying database. But there is no functionality to delete these widgets once they are added.

This can be a problem if the user has added the widget mistakenly.

Affected Product Version:
WSO2SP 4.3.0

[lasanthaS]

This can be implemented by introducing a Widget store to list down all the widgets. If a custom widget is not used in any of the dashboards, then the store will show an option to delete the widget from the store.

[TanyaM]

We have an existing api to delete a generated widget from the database but this api has a bug at [1] as we pass widget type as CUSTOM.

[1] https://github.com/wso2/carbon-dashboards/blob/master/components/dashboards/org.wso2.carbon.dashboards.core/src/main/java/org/wso2/carbon/dashboards/core/internal/WidgetMetadataProviderImpl.java#L100

[niveathika]

This is introduced from SP-4.4.0-M14