Show file history

This introduces the basical possibility to view just the history of one file (#241).

* [ ] Above the table, there should be a header like `File history of <b>{file.relative}</b>` :question: 
* [x] On file change, the table contents should either change or the history should hide completely
* [x] On file click (-> show diffs of changed files), the file should be pre-selected
* [x] If no file is open, we should disable the menu entry
* [x] Use two icons (next to each other) to invoke the corresponding functionality

cc @larz0