Set "ShouldShow=false" when quitting the app using amy method
...Not just the Skip and Finish buttons. This isn't an issue in the Plasma 6.0 version of the app which uses different heuristics to determine whether to autostart the app, but it is an isue for the Plasma 5.27 version which simply checks for the presence of "ShouldShow=False" in the config file.
The fact that we haven't gotten absolute avalance of bug reports about this indicates that nearly all of the people who encounter the welcome app are either clicking on the Skip button, or going through the entire thing and then clicking on the Finish button. So that's nice at least.
Test Plan: remove "ShouldShow=false" from ~/.config/plasma-welcomerc, launch the app, and quit it with Ctrl+Q, Alt+F4, titlebar close button, or Task Manager close button/menu item
BUG: 466475 FIXED-IN: 5.27.3
cc @olib