[REVIEW ONLY] Show file history

This introduces the basical possibility to view just the history of one file (#241).

There are some UI/UX issues left, so please don't merge it yet.
* Above the table, there should be a header like `File history of <b>{file.relative}</b>`
* On file change, the table contents should either change or the history should hide completely
* On file click (-> show diffs of changed files), the file should be pre-selected
* If no file is open, we should disable the menu entry

cc @larz0