Improve mouse selection of code

[mbomb007]

If you try to select the code with the mode to copy it, the line numbers are copied, too, which is annoying. Any code on the left will also be copied. It would be nice if there was an easy way to select and copy code from the patch.

This might be easily fixed if https://github.com/daviddengcn/patch-ext/issues/8 was implemented, because when viewing a unified diff in GitHub or GitLab, only the code is selected.

Here's a diff from this project as an example:

https://github.com/daviddengcn/patch-ext/commit/d28715f50f4ce86ab671dc1647ad5834795a6e43#diff-9af31eabbb7181a962dca45f3e768cbc