Implement a first, simple changelog dialog.

When Rockbox Utility is started for the first time, a new version is started or
the user selected to do so on startup a changelog window is shown.

Change-Id: Ic223e092a09d31ccbbfcd9b973355225cac27632

rbutil/rbutilqt/base/rbsettings.cpp

@@ -31,6 +31,7 @@ const static struct {
     const char* def;
 } UserSettingsList[] = {
     { RbSettings::RbutilVersion,        "rbutil_version",       "" },
+    { RbSettings::ShowChangelog,        "show_changelog",       "false" },
     { RbSettings::CurrentPlatform,      "platform",             "" },
     { RbSettings::Mountpoint,           "mountpoint",           "" },
     { RbSettings::CachePath,            "cachepath",            "" },

rbutil/rbutilqt/base/rbsettings.h

@@ -31,6 +31,7 @@ class RbSettings : public QObject
         //! All user settings
         enum UserSettings {
             RbutilVersion,
+            ShowChangelog,
             CurrentPlatform,
             Mountpoint,
             CachePath,