feat: initial x3-ssh client

- GTK3 GUI with VTE terminal emulation
- Tabbed SSH sessions
- Server management (add/edit/remove)
- SSH key management (generate/import/delete)
- Quick connect bar
- libssh for SSH connections
- autotools build system (configure && make)

configure.ac

@@ -0,0 +1,14 @@
+AC_INIT([x3-ssh], [1.0.0], [x3@sysdaemon.xyz])
+AC_CONFIG_SRCDIR([src/main.cpp])
+AC_CONFIG_AUX_DIR([build-aux])
+AM_INIT_AUTOMAKE([foreign -Wall -Werror subdir-objects])
+
+AC_PROG_CXX
+AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
+
+PKG_CHECK_MODULES([GTK], [gtk+-3.0 >= 3.24])
+PKG_CHECK_MODULES([VTE], [vte-2.91 >= 0.50])
+PKG_CHECK_MODULES([LIBSSH], [libssh >= 0.9])
+
+AC_CONFIG_FILES([Makefile src/Makefile])
+AC_OUTPUT