common/parse-bool.c: make parse_bool() generic

...to avoid multiple versions of a boolean-parser.

- Optionally take a default value
- Return -1 on error
- Rename get-bool.c to parse-bool.c

src/common/meson.build

@@ -3,11 +3,11 @@ labwc_sources += files(
   'dir.c',
   'fd_util.c',
   'font.c',
-  'get-bool.c',
   'grab-file.c',
   'graphic-helpers.c',
   'mem.c',
   'nodename.c',
+  'parse-bool.c',
   'scaled_font_buffer.c',
   'scaled_scene_buffer.c',
   'scene-helpers.c',