Installer should check for presence of ImageMagick

[Fritigern]

After installing Kim5, none of the operations would work. I then remembered something about ImageMagick mentioned elsewhere in relation to Kim3, installed that and that did the trick.

Please add a check for the presence of required packages and only install when all dependencies are satisfied, and show an error (using zenity?) when one or more dependencies could not be found.

Alternatively, include a note in the readme.md about the dependencies.

[robertjak]

KDialog also needs to be installed, on Manjaro use: sudo pacman -S kdialog