Before commit 71b7efd0, `GKeyFile`
incorrectly allowed invalid escape sequences: it would treat the
sequence as a literal, set a `GError`, but not return failure from the
function. So if a caller was explicitly checking for returned `GError`s,
they could detect the invalid escape; but if they were just checking the
function’s return value, they’d miss it.

This is not correct use of `GError`, and the [Desktop Entry
Spec](https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s04.html)
doesn’t allow for invalid escape sequences to be accepted. So it’s wrong
in both ways.

However, the commit above changed this behaviour without realising it,
quite close to the 2.78 stable release deadline. There are numerous key
files in the wild which use invalid escape sequences, and it’s too late
in the cycle to ‘break’ parsing of all of them.

So, for now, revert to the old behaviour for invalid escape sequences,
and give people another cycle to adapt to the changes. This will likely
mean they end up calling `g_key_file_get_value()` rather than
`g_key_file_get_string()`. See
#3098

 for tracking
re-enabling the error handling for invalid escape sequences.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Fixes: #3095
See: #3098