Add check for tabs widget to checkRequirements()
Prevent loading tag manager if checkRequirements() fails
Make tree view and list view each optional with show_tree and show_list
properties
Change createTagTree()'s purpose, add createTagTreeBranch()
Add and implement setupTabs()
Fix undefined variable error
Give #available_tags > div position: relative to help with determining
link positions
Overhaul unselectTag() and availableTagIsVisible()
Include $options array in Tag::setup() parameters
Use $options parameter to pass variables to JS TagManager.init()