Limit number of templates loaded from template directory

When there is a big number of templates in the template directory, in thunar 1.8.x startup will be delayed. In thunar 4.16.x and master e.g. opening the "file" menu will be delayed.

xfdesktop limits the number of templates hard-coded to 16: xfdesktop limits the number of items to 16: https://gitlab.xfce.org/xfce/xfdesktop/-/blob/xfce-4.16/src/xfdesktop-file-icon-manager.c#L358 ... property is never set somewhere, so that is hard-coded.

16 is a bit limited ... I would rather go for lets say 100 ? (500 does not show any delay on my 10 year old system, though I am using a ssd) Well, whatever we choose, I bet there will be somebody using more templates. So probably we need an xfconf variable .. or should we just take a try and wait until someone complains ?

In addition we need an extra-item telling that templates were dropped due to that limitation.

Use this in console for creating template files: for i in {1..500}; do touch "$i"; done

Followup of #463 (closed)