Add Setting for UI Scale Factor

[regulus79]

This PR adds a slider to the setup dialog/settings window, allowing users to set the QT_SCALE_FACTOR from within LMMS. This is done by calling qputenv to set the environment variable before the QApplication starts.

Demo

lmms_ui_scale_factor_demo.mp4

However, there is a problem; I assumed that we wanted to store the scale factor in the config file, but you cannot load the config file until the qApp starts. But you have to set the scale factor env var before the qApp starts for it to take effect. So, I decided to add a temproary qApp object which would be used to load the settings, then set the env var, delete the qApp, and continue with the startup (kind of based on https://stackoverflow.com/a/55408574).

Does anyone have any better idea on how this could be done? Should we even be using the scale factor environment variable for ui scaling?

[regulus79]

I have converted this PR to draft until a better approach can be found which does not require forcefully setting env variables.