Insert SymbolInstance into the current page

[dabbott]

We should display an "Insert" DropdownMenu that will let the user choose a symbol to insert into the current page. This menu displays the names of every SymbolMaster:

After clicking an item in this menu, we should display a preview of the SymbolInstance under the mouse cursor (the preview should be centered on the mouse cursor):

Clicking anywhere on the canvas will then place the symbol at that location. If the clicked point is within an artboard, the SymbolInstance will be added to the artboard's, rather than the page's layers.