Add UI scaling option

[kblaschke]

On some platforms/displays, the auto-detected font size might be a bit too small or large, so we should let the user apply a custom scaling factor.

Since the existing sliders in the UI apply their value immediately, I had to add another one that has an apply button, as moving the sslider causes issues if the UI rescales immediately.

[revmischa]

Do you think something like https://wiki.libsdl.org/SDL2/SDL_GetDisplayDPI could be useful for getting a default scale size?

[kblaschke]

Do you think something like https://wiki.libsdl.org/SDL2/SDL_GetDisplayDPI could be useful for getting a default scale size?

Tried that, but it was super unreliable and gives hugely different results on different platforms, none of which were really useful for font scaling. Would be great if it actually worked because that would enable the app to properly scale fonts based on the current display, as multiple monitors on the same machine can have different DPI, like an internal display of a laptop and an externally connected monitor as a second display.

DPI scaling is a huge issue with any UI framework that works on more than one OS though, ask the Qt devs about that... 😁