Feature: enabled/disabled state

[KabaDH]

This PR is a preview and is not yet ready for production.

This PR introduces a new parameter "enabled" for all CustomDropdown constructors. Setting this parameter to "false" you set the "disabled" state. In the disabled state you can not open the dropdown. Disabled state has its own decoration (disabledDecoration). Disabled state is safe to use with empty input options data.

As a minor feature, this PR introduces parameter "closeDropDownOnClearFilterSearch". Setting this parameter to "true" will cause to close the dropdown upon clicking "Clear filter" cross in any CustomDropdown constructor with search filter.

• [x] Added new fields and adapted existing constructors
• [x] Updated builders to handle "enabled" state
• [x] Performed some tests.