Draft PR: Recently opened files; And opening repeat file dialogs in same directory

[adi-g15]

It's not done, though as said in title, I tried adding a menu option in File, just below Open File, like this, where Recently Opened should be a dropdown like in VS Code's and others Open Recent options, but haven't programmed in qt, tried but didnt find much help in qt docs:

Also the xdg dependency isn't needed, either we can do:

1. Just create and use '~/.config/device-tree-visualiser', instead of BaseDirectory.save_cache_path("device-tree-visualiser")
2. or use current directory for the recents.list file

I will just leave it here, you may just close it. Though have a look at adi-g15@1025c8c, it's mergeable too, for eg. if I opened a dts file from "/home/adityag/os_projects/rpi_linux/arch/arm64/boot/dts/broadcom", and using Open File option once more, I think it's more convenient for the application to open in the same dir, so I may decide to go up a directory maybe or chose another in same dir, but easier than going again.
Just my view, I forked already. Thanks a lot for the application, it helped me in learning and seeing some dts myself (visually much better) 👍

[bmx666]

@adi-g15 I'm not well familiar with Qt, xdg and UI on Linux, I left this functionality from owner.
I implemented "command line" way, which I frequently using like
python3 ./dtv.py /path/to/arch/arm64/some.dts

I'm working on feature to support overlays merge process from command line
python3 ./dtv.py /path/to/arch/arm64/some.dts /path/to/arch/arm64/overlay1.dts /path/to/arch/arm64/overlay2.dts
It should visualize final dts with applied overlays (which are used the reference syntax, without "fragment@X")