Add multi-window support

Previously Alacritty would always initialize only a single terminal
emulator window feeding into the winit event loop, however some
platforms like macOS expect all windows to be spawned by the same
process and this "daemon-mode" can also come with the advantage of
increased memory efficiency.

The event loop has been restructured to handle all window-specific
events only by the event processing context with the associated window
id. This makes it possible to add new terminal windows at any time using
the WindowContext::new function call.

Some preliminary tests have shown that for empty terminals, this reduces
the cost of additional terminal emulators from ~100M to ~6M. However at
this point the robustness of the daemon against issues with individual
terminals has not been refined, making the reliability of this system
questionable.

New windows can be created either by using the new `CreateNewWindow`
action, or with the `alacritty msg create-window` subcommand. The
subcommand sends a message to an IPC socket which Alacritty listens on,
its location can be found in the `ALACRITTY_SOCKET` environment
variable.

Fixes #607.

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/features.md b/docs/features.md
index fd9a9ad2..a97c143d 100644
--- a/docs/features.md
+++ b/docs/features.md
@@ -78,3 +78,9 @@ change in mouse cursor shape, you're required to hold <kbd>Shift</kbd> to bypass
 that.
 
 [configuration file]: ../alacritty.yml
+
+## Multi-Window
+
+Alacritty supports running multiple terminal emulators from the same Alacritty
+instance. New windows can be created either by using the `CreateNewWindow`
+keybinding action, or by executing the `alacritty msg create-window` subcommand.