Improvements to the splash screen

The MuseScore splash screen is rendered using two pre-rendered images, one for low-resolution displays and another for high-resolution displays.

This has the following consequences:

1. The splash screen looks mostly fine on displays that happen to match the exact DPI resolution of the two pre-rendered images, but for anything in between, it ends up looking blurry and/or blotchy. This does not make a good first impression on users.

2. The pre-rendered image files are somewhat large and increase the size of the MuseScore binary distribution by ~1.4 MiB.

3. The pre-rendered images contain “burned-in” text containing the current MuseScore version. That means that the images need to be updated by a designer every time a new version of MuseScore is released.

4. The “burned-in” text on the pre-rendered images is rendered using a font called SFProDisplay. This is an Apple system font that looks out of place on Windows and Linux and also clashes with the font used for the status messages that are displayed in the middle of the splash screen.

5. The splash screen no longer has a special design for developer builds as it used to in older versions of MuseScore.

This problems can all be solved by rendering the splash screen on the fly using vector-based graphics instead of using pre-rendered images.