First-time setup dialog

[askmeaboutlo0m]

When a new person starts using Drawpile, they usually have to do quite a bit of fiddling around in the preferences to get the program looking and feeling how they want. We should instead present them with a guided setup that does at least the following:

• Before even starting Drawpile proper, let them pick the most important parameters: their language, UI style (maybe restricting this to Bright, Dark and Darker) and maybe other stuff that normally requires restarting the application to apply.
• Maybe give them a selection of what program they're coming from so that we can set up their general settings. For example, if they come from Krita, they will almost certainly want E to toggle erase mode, not switch to a different eraser brush.
• Provide guided setup for the tablet driver, smoothing and pressure curve.
• Let you pick your initial application layout, since the "Default" layout is okay for small screens, but wastes a lot of space on large ones.

[MorrowShore]

Could potentially be added to the start screen instead #1345
Related to #1206