Prevent confusion around _thunar_return_val_if_fail and _thunar_return_if_fail

Every now and than it happens that bugs / crashes in distro-packed thunar cannot be reproduced easily, because they only occur when _thunar_return_val_if_fail and _thunar_return_if_fail are disabled. (When building from source, these calls are enabled by default)

To disable them, it is required to build with --disable-debug. Here the related code: https://gitlab.xfce.org/xfce/thunar/-/blob/a90e946ae945e1d3ae83e9e1b4f37e667d1f24b1/thunar/thunar-private.h#L29

Having these guards only active when building from source in most situations is not desired, leading to additional if-checks or g_return_val_if_fail / g_return_if_fail calls.

Till now I did not face any performance penalties introduced by the guards, so I would suggest to just have them always active.

Another option would be to rename them to _thunar_debug_return_val_if_fail and _thunar_debug_return_if_fail in order to at least make it more clear that they are debug-only. Though I would prefer to have option 1: always-enabled.

@andreldm , @Tamaranch , @SKefalidis , @MShrimp4 : What do you think ? Would be great to hear your opinion on this suggestion !

Related: #808 (closed)