Implement the session lock protocol

Currently implemented:

- Locking and unlocking.
- Display above all desktop layers, including OVERLAY (but below
  DWIDGET).
- Waiting for the client to display locked surfaces before
  declaring the screen to be locked. If the client does not
  display all surfaces, locks anyway after a timeout.
- Multiple outputs, including outputs being added/removed while
  locking or locked.
- Leaving the screen locked if the client crashes (but allows a
  new client to come in and lock the screen).

TODO:
- Deal with outputs being resized.
- Properly display text if the screenlocker crashes. Currently it
  just displays two a big "explosion" emoji.
- Use a view instead of a plain wlr_surface_node?

Tested with swaylock.

Fixes #1494
Fixes #1358

metadata/meson.build

@@ -42,3 +42,4 @@ install_data('shortcuts-inhibit.xml', install_dir: conf_data.get('PLUGIN_XML_DIR
 install_data('wsets.xml', install_dir: conf_data.get('PLUGIN_XML_DIR'))
 install_data('wayfire-shell.xml', install_dir: conf_data.get('PLUGIN_XML_DIR'))
 install_data('xdg-activation.xml', install_dir: conf_data.get('PLUGIN_XML_DIR'))
+install_data('session-lock.xml', install_dir: conf_data.get('PLUGIN_XML_DIR'))

metadata/session-lock.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0"?>
+<wayfire>
+	<plugin name="session-lock">
+		<_short>Session Lock Protocol</_short>
+		<_long>An implementation of the ext-session-lock-v1 protocol. Provides more secure screen locking.</_long>
+		<category>Utility</category>
+	</plugin>
+</wayfire>

plugins/protocols/meson.build

@@ -1,5 +1,6 @@
 protocol_plugins = [
-  'foreign-toplevel', 'gtk-shell', 'wayfire-shell', 'xdg-activation', 'shortcuts-inhibit', 'input-method-v1'
+  'foreign-toplevel', 'gtk-shell', 'wayfire-shell', 'xdg-activation', 'shortcuts-inhibit',
+  'input-method-v1', 'session-lock'
 ]
 
 all_include_dirs = [wayfire_api_inc, wayfire_conf_inc, plugins_common_inc]