Update Content Handlers path
Load all content handlers in single `script` element
Set init properties on `SLB_Base_Object` directly
* Add: `set_props()` (from `Content_Handler` class)
* Add: `set_id()`
* Update: Constructor sets properties as well
Add `SLB_Template_Tag` class
Add `SLB_Template_Tags` class
(Based off of `SLB_Content_Handlers` class)
Enable template tags to be loaded from server-side

Server
* Add: Initialize template tags collection

Client
* Update: `add_template_tag_handler()` accepts URI to handler definition
* Remove: Default template tag handlers
Register default template tags (via server)

* Add: `item` tag handler
* Add: `ui` tag handler
Cleanup: Add inline documentation to `Base_Collection` class
Merge branch 'register_template_tags' into develop
Update changelog in readme (for 2.0b7.3)