Impl. clearable dropdown

[mvorisek]

simillar to 6fcc1f2

fix #572
fix #2026

[mkrecek234]

Hi @mvorisek With this PR, Lookup fields always get a clearable icon if they are either nullable or not required. I am of the opinion that

a) clearable should only be activated by default if the field is not required and is nullable at the same time
b) it should still be possible to override this default behaviour using setDropdownOption('clearable', false) - currently not possible as it is forced on rendering.

Furthermore I saw that on a disabled/readOnly field with `multiple = true, the clearable icon was present allowing the erase the dropdown field.

Let me know if you agree, then I can work on a PR to suggest a change.

[mvorisek]

Hi Michael, "almost always clearable" - what are the usecases? User should be able to revert any chenge he did on the UI side, so after selecting an option it should be always possible to clear it. The validation on send is of course always working.

"multiple" - if there is an issue, PR with testcase highly welcomed!