[Prototype] Settings bar to replace context menu for options

Adding the settings bar as discussed in #2714. Not yet to merge, but to share thoughts and to further improve this such that it can be merged later.

Remaining tasks:
- [ ] Only slide the viewerContainer, not the mainContainer (couldn't get this to work properly, because if I did this, the bar would move into the viewerContainer, but would not move the viewerContainer itself).
- [] Let UX designers replace the settings toolbar icon (I'm not quite happy with it as it isn't completely the same style as the rest of the icons) and let them check if the icons for first/last page are alright. We will also need icons for other options, like rotatie clockwise/counterclockwise, etc.
- [] See if UX designers have improvements for the style of the bar as it is now.
- [] Move all existing context menu features to the options bar.
- [] Add new strings to the language files
- [] Code evaluation (does the CSS/JS/HTML conform pdf.js coding standards; hard for me to decide, so I'd like to hear your comments about this)

Feel free to comment on this and report things that I missed here. :)