Add a settings modal

[GameGodS3]

Settings modal can be independent or just a modification of the existing DropPoint instance.

Things planned to be in the settings modal:

• Enable/disable Splash Screen
• Start Minimised
• Change Shortcut Keys
• Don't minimise after file transfer
• Click behaviour of tray icon (Single Click, Double Click)
• Switch between copy and move modes

Settings could be stored in a separate JSON file

[leidichen]

• Remember window position

[dairiki-ren]

• disable the launching shortcut because shift-tab can sometimes be useful :)
  Edit: Actually, nevermind. I started a tiny pull request for a temporary fix for this issue. But I hope that a more permanent solution (e.g. customizable instance shortcut) will be introduced in the next update.