Added functionality to render layers into separate files, as well as to "solo" layers in the timeline.

We have added the functionality to render layers into separate files (this option is enabled when activating a checkbox in the advanced render settings), these files follow the naming convention "filename_layer-priority_layer-name.file-extension". As well as this functionality, we have added the ability to "solo" a layer in the timeline, which is done by muting and/or hiding all other layers in the timeline. This is found under the layer options dropdown.