Added option to remove cursor from sdl window

[JannisKirschner]

Hey folks,
Ever opened the frontend but couldn't figure out where to best place the os cursor so it's not as visible?
I created a commandline option to remove it if preferred for an unobstructed experience.

[kblaschke]

Actually a good idea, as the cursor is always visible in windowed mode as of now. I'd recommend waiting a bit for my Dear Imgui branch to be merged to master, which I'm going to do soon. There were lots of changes, and the mouse pointer must be visible while the UI is open. Currently, the mouse pointer is hidden when the UI is closed, no matter if the app is in fullscreen mode or not, but when toggling between windows/fullscreen mode, the pointer will be shown again in windowed mode, which isn't optimal.

I guess your suggested command-line option (plus adding a configuration/UI option) to permanently hide the mouse pointer in windowed mode is definitely the way to go!

That said, if you're patient until the merge is done, you can just rebase (or redo) the changes in this PR and add the proper options to the configuration file and UI's settings window as well.

To familiarize yourself with the new UI before the merge, feel free to check out and build the branch linked above!

Edit: Actually just opened PR #76 to merge the branch!

[kblaschke]

The initial UI code has just been merged - please rebase and see what you can improve!