Add setting for max image width when viewing .note files

[james-xli]

Per issue #22, I took a stab at adding a max px width setting in the settings pane:

Set to 500 px as shown above, my test note looks like this:

Notes are still responsive to window size below the max width.

The default value is 1400px, which is nearly the max width of a note (both A5X and A6X2) in portrait orientation. This is the same as the preexisting behavior. That same test note looks like this at the max 1400px width:

I am pretty new to contributing on Github so I would like any and all feedback, thanks!