Update CMake configuration to compile against Plasma 6

[RedSQL]

I have updated CMake configuration and now it should compile fine against KF6/Qt6, including it's Plasma5Support modules!

I don't exactly know what the cmake stuff compiles... But it compiles at least. Oops! Right, it compiles files for use in /usr 😅

• I have updated CMake configuration in the root directory and the plugin directory.
• I also have changed backend.cpp so compiler would no longer complain about endl usage instead of \n.
• The dev-build.sh script should now work.
• I have made dev-build.sh script slightly more robust, now it should not prompt for sudo if cmake command fails.

This should not conflict with #166

This resolves #163 (hopefully)

[blackadderkate]

Seems to compile on my Virtual Machine development box, but "make" seems to return 1 rather than zero for some reason (so it won't install) although I can't see any obvious errors. OpenSuse Tumbleweed is continuously updated, so it's possible there's a file conflict or outdated library - I'll update everything and try again.

[RedSQL]

Seems to compile on my Virtual Machine development box, but "make" seems to return 1 rather than zero for some reason (so it won't install) although I can't see any obvious errors. OpenSuse Tumbleweed is continuously updated, so it's possible there's a file conflict or outdated library - I'll update everything and try again.

Oops! Sorry, I don't think I was meant to use -z $? check!

I have addressed the issue in #169 with $? -eq 0 - which properly should fail if $? is non-zero, and if the build succeeds it should run sudo make install