LibCore: Refactor a version-reading utility

`ArgsParser` and `AboutDialog` had the same procedure to read the
version from `/res/version.ini`. Now they use the `SERENITY_VERSION`
string by default.

This commit refactored the version-reading utility to the new
`Core::Version` namespace.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -31,6 +31,7 @@ set(SOURCES
     Timer.cpp
     UDPServer.cpp
     UDPSocket.cpp
+    Version.cpp
 )
 
 serenity_lib(LibCore core)

Userland/Libraries/LibCore/Version.cpp

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2021, Mahmoud Mandour <ma.mandourr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/ConfigFile.h>
+#include <LibCore/Version.h>
+
+namespace Core::Version {
+
+String read_long_version_string()
+{
+    auto version_config = Core::ConfigFile::open("/res/version.ini");
+    auto major_version = version_config->read_entry("Version", "Major", "0");
+    auto minor_version = version_config->read_entry("Version", "Minor", "0");
+
+    StringBuilder builder;
+    builder.appendff("Version {}.{}", major_version, minor_version);
+    if (auto git_version = version_config->read_entry("Version", "Git", ""); git_version != "")
+        builder.appendff(".g{}", git_version);
+    return builder.to_string();
+}
+
+}

Userland/Libraries/LibCore/Version.h

@@ -12,4 +12,6 @@ namespace Core::Version {
 
 constexpr StringView SERENITY_VERSION = "Version 1.0"sv;
 
+String read_long_version_string();
+
 }