Add labnag

Based on swaynag (https://github.com/swaywm/sway/tree/master/swaynag)

Copied at commit:
03483ff370

Contains the following modifiations:

- Some functional changes including:
  - Disable exclusive-zone by default (Written-by: @Consolatis) and add
    command line option -x|--exclusive-zone
  - Add close timeout (Written-by: @Consolatis) and -t|--timeout option
  - Use index of button (from right-to-left) for exit code
  - Disable reading from config file and remove associated --type option
- Refactoring including:
  - Use wlr_log() instead of the log.{c,h} functions
  - Use wl_list instead of sway's list.c implementation
  - In the pango wrapper functions, use glib's g_strdup_vprintf() rather
    than the original stringop.c functions
- Align with labwc coding style to pass checkpatch.pl
- Re-licenced from MIT to GPL-2.0, and add Copyright notices for original
  authors

v2

- Remove option -s|--dismiss-button and the default "X" button. To get
  such a button, "-Z X :"
- Remove options -b and -z because there is no requirement to run
  in a terminal.
- Remove *-no-terminal from options --button and --button-dismiss because
  commands are now always run directly without a terminal.

v3

- Allow -B/-Z options without action-argument
- Invert button order of -B/-Z so that `labnag -m foo -Z x -Z y -Z z`
  results in three buttons with "x" furthest to the left, and "z" on the
  right (rather than the other way around).
- Use signalfd() to prevent race conditions on SIGTERM

v4

- Limit number of stdin lines to 200 to avoid hogging CPU

Co-Authored-by: tokyo4j

docs/meson.build

@@ -7,6 +7,7 @@ if scdoc.found()
     'labwc-config.5',
     'labwc-menu.5',
     'labwc-theme.5',
+    'labnag.1',
   ]
   foreach manpage : manpages
     markdown = manpage + '.scd'