Add asynchronous loading for the initial model

[grapoza]

This enhancement will provide a way to load the initial model asynchronously, similar to how a node can currently load its children. This came from #192.

Potential ideas include:

• allow initial-model to be a function instead of an array, expecting an asynchronous function that would itself return an array of data
• this could be its own property matching the way the TreeViewNode handles it

This can probably reuse the treeViewNodeChildrenLoad events that already fire for subnodes, though I'd probably want to rename the method; treeViewNodeAsyncLoad?
The isMounted data/prop should probably become isTreeDataLoaded or something more descriptive, since that shouldn't get switched to true until the initial model has loaded.