Don't remove newlines when parsing config, menu and XBM

Removing newlines in rc.xml and menu.xml caused parser error with
following content:

<!--
 -
 - Some comments
 -
-->

...though it is a valid XML.

Let's not do that. I moved `grab_file()` to `buf.c` and renamed it to
`buf_from_file()`, because it now directly touches `struct buf` and
I don't like having a source file only for one function.

src/common/meson.build

@@ -6,7 +6,6 @@ labwc_sources += files(
   'fd-util.c',
   'file-helpers.c',
   'font.c',
-  'grab-file.c',
   'graphic-helpers.c',
   'lab-scene-rect.c',
   'match.c',