More right-click options

[Akuli]

In the directory tree, after right-clicking a directory it would be nice to have an option "Open in terminal", instead of having to open a new terminal and cd to the directory. It's already possible to run commands in a terminal, and we can probably reuse much of the code.

~~It would be also nice to have a "Copy full path to clipboard" option, so that I can paste the path into some other program instead of browsing or typing it manually.~~ (this part is now done)