Add option to cut off borders

[ericschdt]

Issue Summary

Some documents have large white borders at the left and right edges. In order to have to text filled the screen, you have to zoom. However, 1) this is an extra step and 2) you end up with possible additional vertical scrolling.

For instance, Okular has the neat feature to simply cut off the borders. It would be interesting to have it here too!

Specifications

• App Version: Latest F-Droid