Improve dropdown

[tomsmalley]

This is a rewrite of the dropdown module. Main API change is the addition of the Event argument to dropdown. Improvements:

• You can now update the value
• Searchable dropdowns are now first class rather than being in a separate (worse) function
• Searchable dropdowns are keyed by default. The old text-only search (with allow additions) dropdown behaviour is available in textDropdown.
• Dropdowns can be initialised with a custom wrapper (e.g. button, label, etc), this replaces the questionable config values that were used previously
• Dropdowns can open upwards
• No longer capture the space keypress event (matches Semantic-UI behaviour)