* GH-309: When opening a file via a GUI action, check whether the file is already open.
    - Open in a tab or split: select the tab/split
    - Open in another (GUI) window: let NeoVim handle it.
* GH-239, GH-312: Turn on font smoothing such that the "Use LCD font smoothing when available" setting from the General system preferences pane is respected.
* GH-270: Make line spacing configurable via the "Appearances" preferences pane.
* Bugfix: The command line tool `vimr` sometimes does not open the files in the frontmost window.