Simplify _fm_vfs_search_resolve_relative_path().

We already have GFile of directory where the file was found
so we can simply run against it instead of reforging the path.
Don't allow drop files onto R/O file system.
[#2995549]Change text&icon on "Delete" context menu item when Shift is pressed.

If trashing is allowed the pressing the Shift should affect the "Delete"
context menu item - show "Trash" when Shift not pressed and "Remove" when
it is pressed. Tracking key press on widget is enough to do that.
Fix cast when casts are done from 32-bit to pointer.

Also change some obsolete comments in code.