Dont check executable bit/trusted flag for .desktop files in $XDG_DATA_DIRS

From #50 (comment 60122) (FYI @MichaIng)

Generally I agree that desktop launcher from anywhere shouldn't be trusted by default, although a setting to do so would be nice, also to align with other desktop environments.

However, there is one bug in the current Thunar behaviour:

* /usr/share/applications is a trusted location: Every user can launch .desktop files from there without warning, manually from file explorer or via panel applications menu.
* If however a symlink is created from \~/Desktop/some.desktop to /usr/share/applications/some.desktop, it shows the warning, and the user cannot mark it as executable since it doesn't have write permissions, of course.
* I suggest to also trust symlinks to desktop launchers in trusted locations, especially the default one in /usr/share/applications. Packages do never ship those launchers with executable bit, but it makes much sense (and AFAIK is common practice) to symlink those to individual user desktops, like Kodi, htop, Chromium etc. It would be great if this worked without the permanent warning, as I cannot see any security downside. Since by default only root has write access to /usr/share/applications and any file inside, it is the even more secure solution, compared to creating own executable desktop launchers right within the users home dir, which in theory can be overwritten with user privileges.

I hope this makes sense. Best regards, Micha

Reading to the topic: https://askubuntu.com/q/419610/237592

The following got already fixed:

In addition with current thunar/exo master, there seems to be a problem with opening the preferences dialog for .desktop files which are not writable. E.g. open thunar /usr/share/wayland-sessions and try to set the executable flag for weston.desktop.

At the first try thunar segfaults on setting the flag. At the second try thunar segfault already on opening the preferences.