Fix disabling of zoom buttons and add camel case button names

If the user zooms in (or out) until the maximum (minimum) zoom level is reached, the corresponding zoom button will be disabled. If the user now chooses a predefined zoom level in the drop down box, for example "Page Fit", the disabled zoom button won't be re-enabled.
This PR fixes the above, and also changes a couple of related button names to camel case. Lastly, this PR also replaces a couple instances of `querySelector` with `getElementById`.