Add the missing libfm-qt files

[kettle-7]

These files are taken directly from https://github.com/lxqt/libfm-qt, and contain C++ files and headers for handling file operations.

Somehow @probonopd managed to compile Filer without them, I'm assuming that's because he had the files already in /usr/include.

Anyway, these can now be modified to help fix issue #53.

[probonopd]

@linuxkettle I really doubt that this is necessary. If we would really need this, how come that we can compile Filer without it like this:
https://github.com/helloSystem/Filer/blob/master/.github/workflows/build.yml

Somehow @probonopd managed to compile Filer without them, I'm assuming that's because he had the files already in /usr/include.

In this case they would have to come from one of the packages specified in the yml file. They can't appear out of thin air ;-)

[kettle-7]

That is strange. If Filer doesn't need these headers to compile, where could it get the function definitions from? I forgot to run grep in /usr/local/include, but when I did, it came up with exact copies of these files.

I'm no C++ expert, but I do know two things: files can't appear out of thin air (so Filer mustn't need these files) and functions need definitions (so Filer must need these files)

Confusing!

[probonopd]

So we both need to keep looking!