add session save and session load functionality

New [key-bindings]:
- session-save: captures cwd and foreground process argv to ~/.local/share/foot/state/{name}.json
- session-save-secure: prompts for a password, encrypts the scrollback with argon2id + XChaCha20-Poly1305 (libsodium) and writes it to {name}.scrollback.enc(stores up to 1Mb scrollback buffer).
- session-load: a minimal fuzzy picker that displays saved sessions (both secure and vanilla), UI piggybacks on search bar subsurface. use arrows to navigate and delete to delete a previously saved session.

meson.build

@@ -162,6 +162,7 @@ endif
 
 tllist = dependency('tllist', version: '>=1.1.0', fallback: 'tllist')
 fcft = dependency('fcft', version: ['>=3.3.1', '<4.0.0'], fallback: 'fcft')
+libsodium = dependency('libsodium')
 
 wayland_protocols_datadir = wayland_protocols.get_variable('pkgdatadir')
 
@@ -330,6 +331,8 @@ executable(
   'reaper.c', 'reaper.h',
   'render.c', 'render.h',
   'search.c', 'search.h',
+  'session.c', 'session.h', 'session-prompt.c',
+  'session-crypto.c', 'session-crypto.h',
   'server.c', 'server.h', 'client-protocol.h',
   'shm.c', 'shm.h',
   'slave.c', 'slave.h',
@@ -342,7 +345,7 @@ executable(
   'xkbcommon-vmod.h',
   srgb_funcs, wl_proto_src + wl_proto_headers, version,
   dependencies: [math, threads, libepoll, pixman, wayland_client, wayland_cursor, xkb, fontconfig, utf8proc,
-                 tllist, fcft],
+                 tllist, fcft, libsodium],
   link_with: pgolib,
   install: true)