Fii's questions

On my Kubuntu 18.10 19.04 system, I have a software installed (in this case: the Arduino IDE), which I would like to completely uninstall.

• I cannot remember how I installed it, but it does not show up in dpkg --list or the software center and apt-get purge won't find it. I'm pretty sure it was installed without any package manager.
• I downloaded the current (newer than installed) version from the developer's website (https://www.arduino.cc/en/Main/Software), which includes an unistall.sh script. But the only effect it seems to have is that the launcher shortcut (which is still there, for that matter) doesn't work anymore - the software can still be run from terminal (by simply typing arduino).

What, generally, are the steps to take to manually uninstall software from my system? I assume it's mainly deleting files, but how do I locate them all? Are there any databases I need to un-register the software from?

I'm using Kubuntu 18.10 and have the default file manager, Dolphin, set up quite nicely with servers, favorites etc. in the sidebar.

Now, whenever I want to open a file or folder from within a program, this file picker opens:

Now, many programs open this file/folder picker instead of the Dolphin file picker:

EDIT: This happens for example in Sublime Text and Firefox. Preinstalled programs (e. g. Spectacle or Kate) seem to actually do display a Dolphin file picker!

Is there any way to replace this with the Dolphin file picker, or at least get my favorites etc. into whatever this program is?