chore(3p/exwm): Subtree EXWM ... again

Upstream EXWM is now sort-of unmaintained[0], but some important PRs
are pending[1] which I would like to absorb here somehow.

[0]: https://github.com/ch11ng/exwm/issues/845
[1]: https://github.com/ch11ng/exwm/pull/848

git-subtree-dir: third_party/exwm
git-subtree-mainline: 7894d7e178
git-subtree-split: 0368127976
Change-Id: I078358c0ea1d2bac99111748ce9c137d2804ac4e

third_party/exwm/README.md

@@ -0,0 +1,21 @@
+# Emacs X Window Manager
+
+EXWM (Emacs X Window Manager) is a full-featured tiling X window manager
+for Emacs built on top of [XELB](https://github.com/ch11ng/xelb).
+It features:
++ Fully keyboard-driven operations
++ Hybrid layout modes (tiling & stacking)
++ Dynamic workspace support
++ ICCCM/EWMH compliance
++ (Optional) RandR (multi-monitor) support
++ (Optional) Builtin system tray
++ (Optional) Builtin input method
+
+Please check out the
+[screenshots](https://github.com/ch11ng/exwm/wiki/Screenshots)
+to get an overview of what EXWM is capable of,
+and the [user guide](https://github.com/ch11ng/exwm/wiki)
+for a detailed explanation of its usage.
+
+**Note**: If you install EXWM from source, it's recommended to install
+XELB also from source (otherwise install both from GNU ELPA).