Optimize our Tasks

[pictos]

After Uno.Resizetizer releases we found some bad behavior on our MSBuild tasks, some of them didn't run when they should, and it causes some issues like missing splashScreen or app icons. To fix that we should re-organize our Targets and Tasks in a way that they will run more efficiently.

Some things to keep in mind:

• Any task that creates images should only run when input/outputs change
• Any task that modifies project properties (including adding files to the package) should always run