Autogen: Add detection of UI file changes to the pre-build VS case

This is achieved by adding a rule file which carries the UI files as dependencies but performs no meaningful command. Its output path points to a timestamp file which is instead touched by the pre-build command that runs autogen. The rule file therefore triggers the build if any of the files have been changed more recently than the last autogen run.

Fixes: #17959
Fixes: #18741
36 jobs for !5999 with vs-autouic-on-ui-file-change in 76 minutes and 26 seconds (queued for 2 seconds)
merge request