icon-loader: add a new loader for XPM icons, used by many legacy X11 apps

Adapted from gdk-pixbuf, see the original at:

  https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/blob/master/gdk-pixbuf/io-xpm.c

rgb.txt is from X.org, see:

  https://gitlab.freedesktop.org/xorg/app/rgb/-/blob/master/rgb.txt

Differences from the gdk-pixbuf version:

- GdkPixbuf replaced with struct lab_data_buffer
- Progressive and in-memory loading code removed
- Two functions that had separate BSD copyright rewritten
- Stores colors as ARGB32 earlier in the decoding process for efficiency
- Limited to 1024x1024 px and 1024 colors to prevent abuse
- Uses struct buf to build strings, in place of manual g_new/g_realloc
- Uses xzalloc/xznew_n for other memory allocations
- Uses g_strlcpy in place of banned strcpy/strncpy/strncat
- Uses standard C types (int, bool, etc.) in place of the GLib ones
- Follows labwc coding style (whitespace, braces, letter case, etc.)
- Et cetera ...

v2: add Perl fixes from @domo141

src/icon-loader.c

@@ -10,6 +10,7 @@
 #include "config.h"
 #include "icon-loader.h"
 #include "img/img-png.h"
+#include "img/img-xpm.h"
 
 #if HAVE_RSVG
 #include "img/img-svg.h"
@@ -211,7 +212,7 @@ icon_loader_lookup(struct server *server, const char *app_id, int size, int scal
 #endif
 		break;
 	case SFDO_ICON_FILE_FORMAT_XPM:
-		/* XPM is not supported */
+		img_xpm_load(ctx.path, &icon_buffer);
 		break;
 	}