Make selectable property more configurable: false, true/'multi', 'single'. Also...

[pavelav]

... require Ctrl to deselect a row to match the declared file manager behavior.

[pavelav]

The idea to support different types on one property was looked up in another component (vue-js-modal), however thinking more about it, it may be needless complexity in the case of selectable property. See the next commit for a simpler alternative using just the String type and the following values: undefined, 'single' and 'multi'. To make things more explicit undefined could be change to 'no' I guess. Please have a look. Thanks!