Support for Music Sheets

Request in forum: https://forums.openlp.org/discussion/3417/feature-request-sheet-music-in-stage-view

In the forum post it is suggested to show the sheet as part of the stage view. An alternative approach could be it create a separate web-based view that only shows sheet music.

It could be implemented by making it possible to attach sheets to songs, in the same way media files can be attached. The formats supported could be images, PDFs and MusicXML. These formats has the advantage that they be rendered in a webview. PDF can be rendered by PDF.js.
MusicXML can be rendered by these:
https://code.google.com/p/score-library/ (web)
https://github.com/0xfe/vexflow (generel web node renderer)
https://github.com/ringw/vexflow/tree/musicxml (vexflow with musicxml support)
http://wim.vree.org/js/