add cli flag to open files tab with selected file

[laktak]

This Pull Request fixes/closes #2510.

It changes the following:

• adds a --file cli flag to open the files tab with the given file already selected

I followed the checklist:

• I added unittests
• I ran make check without errors
  • I can't run it locally because of Build Failure on Arch Linux rust-onig/rust-onig#203
• I tested the overall application
• I added an appropriate item to the changelog

[laktak]

I don't do Rust very often so I'm quite happy that it works ;)

However while I followed the guidelines as best as I could, I hope you can help with this error from clippy:

error: this function has too many arguments (8/7)
   --> src/main.rs:214:1

[laktak]

Is that OK?

#[allow(clippy::too_many_arguments)]
fn run_app(