Show the rename and save confirmation checkbutton

The check buttons were added to the message area of the GtkMessageDialog
but they were not shown.
Remove some unused-but-set variables
Return a value where it is required to do so
Use TRUE and FALSE for Main_Stop_Button_Pressed

Main_Stop_Button_Pressed is defined as a gboolean, so use it as such.
Set parent window of about dialog to main window
Avoid use of AC_CYGWIN and AC_MINGW32

Use AC_CANONICAL_HOST and AS_CASE instead.
Fix some more set-but-unused warnings
Remove further unused and set-but-unused variables
Use stock GTK+ icons in a few more places

Partially fixes bug 689418.
Use icons from the icon naming specification

Try to use freedesktop.org Icon Naming Specification names for icons in
preference to those that EasyTAG installs by default.
Fix missing prototype warnings

Additionally, make the missing prototypes warning an error in the list
of compiler warnings in configure.ac.
Use gboolean for returning TRUE and FALSE
Fix an uninitialised variable warning
Use g_file_new_tmp() for creating temporary file

Add a requirement on GIO 2.32 in configure.ac.
Add static to many declarations

Use the Samba findstatic.pl script to find functions which should be
static, and mark them as such. Remove unused code, as the compiler gives
warnings about unused static functions.
Use g_return_if_fail() and g_return_val_if_fail()
Use G_OS_WIN32 to contain Windows code

WIN32 was used previously, but was no longer set by the configure
script.
Surround Browser_Win32_Get_Drive_Root with ifdef

Only enable the win32-specific function if G_OS_WIN32 is defined.
Remove some unused prototypes
Fix const usage for id3_tag structure parameters

The structs are passed to id3_tag_render(), which accepts a const
pointer to a non-const id3_tag.