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/buf.c

@@ -6,6 +6,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
+#include <wlr/util/log.h>
 #include "common/macros.h"
 #include "common/mem.h"
 #include "common/string-helpers.h"
@@ -203,3 +204,37 @@ buf_move(struct buf *dst, struct buf *src)
 	*dst = *src;
 	*src = BUF_INIT;
 }
+
+struct buf
+buf_from_file(const char *filename)
+{
+	struct buf buf = BUF_INIT;
+	FILE *stream = fopen(filename, "r");
+	if (!stream) {
+		return buf;
+	}
+
+	if (fseek(stream, 0, SEEK_END) == -1) {
+		wlr_log_errno(WLR_ERROR, "fseek(%s)", filename);
+		fclose(stream);
+		return buf;
+	}
+	long size = ftell(stream);
+	if (size == -1) {
+		wlr_log_errno(WLR_ERROR, "ftell(%s)", filename);
+		fclose(stream);
+		return buf;
+	}
+	rewind(stream);
+
+	buf_expand(&buf, size + 1);
+	if (fread(buf.data, 1, size, stream) == (size_t)size) {
+		buf.len = size;
+		buf.data[size] = '\0';
+	} else {
+		wlr_log_errno(WLR_ERROR, "fread(%s)", filename);
+		buf_reset(&buf);
+	}
+	fclose(stream);
+	return buf;
+}

src/common/grab-file.c

@@ -1,36 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0-only
-/*
- * Read file into memory
- *
- * Copyright Johan Malm 2020
- */
-
-#define _POSIX_C_SOURCE 200809L
-#include "common/grab-file.h"
-#include "common/buf.h"
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-
-struct buf
-grab_file(const char *filename)
-{
-	char *line = NULL;
-	size_t len = 0;
-	FILE *stream = fopen(filename, "r");
-	if (!stream) {
-		return BUF_INIT;
-	}
-	struct buf buffer = BUF_INIT;
-	while ((getline(&line, &len, stream) != -1)) {
-		char *p = strrchr(line, '\n');
-		if (p) {
-			*p = '\0';
-		}
-		buf_add(&buffer, line);
-	}
-	free(line);
-	fclose(stream);
-	return buffer;
-}

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',