Fix item sorting by modified datetime

[walkeras]

Existing sortfunc() was returning NaN on Gnome 44.2. Updated to use proper GDateTime compare() function.

[bananenfisch]

I've changed it this way (may be faster than the compare method): https://github.com/bananenfisch/RecentItems/commit/5761560baf5c7fa1c0087c39c7613822de4fa390