Getting the existing symbols could be easier

[mathieudutour]

Right now we iterate through all the pages and then all the layers to get the symbol masters.

There is a convenient method on MSDocumentData called allSymbols and we should just use it.

[mathieudutour]

It's actually not that simple: injectSymbols reset the Symbols page and render all the symbols that we found. So if we use allSymbols, it will bring the symbols from other pages as well.

Probably need to tackle #316 at the same time

[nampdn]

It would be great if we could retrieve all of the available symbols into an array. Currently in v2 I couldn't find a way to achieve this.