feat: settings window

[oliverschwendener]

The goal of this PR is to solve #1119 and have a separate window for the settings.

Work in progress...

Open tasks:

• Add ability to open settings from tray icon context menu
• Add ability to open settings from ueli extension search results
• Add ability to open settings from keyboard shortcut
• Add ability to open settings from "browse extensions"
• Prevent windows from being destroyed
• Prevent hiding of search window on blur when settings window is open and focused
• Reduce memory usage by making sure settings window is destroyed when not used
• Extract common window management behavior to separate module
• Add correct window icon
• Fix bug that prevents the theme from being reactive
• Test app in dev mode on:
  • Windows
  • macOS
  • Linux
• Test packaged app on:
  • Windows
  • macOS
  • Linux