Switch to GFile for I/O operations

• First commit: basic switch to GFile for I/O operations, mainly code refactoring, but fixes #4 (closed) (see #4 (comment 19412)).
• Second commit: lets g_file_replace_contents() check if the file has been externally modified, handles errors as they come (thread safe approach, as recommended in the documentation).
• Third commit: likewise, lets g_file_load_contents() check if the file exists.
• Fourth commit: grays out save action when the file is not modified in the buffer.
• Fifth commit: adds an option (GSettings only, disabled by default) to create a tilde-backup file on saving (fixes #27 (closed)).
• Sixth commit: file monitoring (enabled by default, can be disabled via GSettings, fixes #75 (closed)).