[FEATURE] Add file drop support

[kblaschke]

On all desktops, it's common practice to drop files and folders onto an application. projectMSDL should also support drop events and use them to load presets amd preset dirs.

Depending on the drop location, e.g. the main window, the preset playlist editor or the settings dialog, the dropped files should ideally be handled in a way the user expects. For example if dropped onto the main window, a preset should be played immediately, but if dropped onto the playlist editor, it should be added to the list.

How well the above drop locations can be supported by SDL and ImGui needs to be determined though.