[Feature] UI-Plugin-SelectBox: Configurable Title and updated Readme

Changes in this PR:

- Enhanced the selectbox header to be set via `<select title="Select stuff...">` whilst maintaining the original behaviour (name & id attribute fallback when title attribute is not set)  
- slightly refactored the affected code for better legibility ("clean code" philosophy) and removed unused vars  
- Updated README to reflect the new feature as well as the (apparently undocumented) [`$.ui.selectBox`]()  
- fixed markdown numbering & indendation in README  

I guess a more streamlined approach would be to read the title from a corresponding form label, but as that may add quite some complexity, be confusing (what if there are two labels/what if there's no label/what takes precedence) or maybe break backwards compatibility, i refrained from that and sticked to this much simpler solution.

Hope it's considered useful and doesn't have any other side effects i may have overlooked.