Multi-monitor support

[JezerM]

Based on nody-greeter Multi-monitor support.

Changes

• Refactor browser.py:
  • Added a BrowserWindow class to abstract windows in a similar way of what Electron.js does.
  • Make use of Application.screens() to create a BrowserWindow for each screen/display
  • Devtools, menubar, qdock, and keybindings are now managed by BrowserWindow instead of Browser.
• Bridge objects are connected to each window.
• Added index.yml support (from Nody Greeter) to load theme config and set both primary and secondary HTML to each window.
• Now, the code should be easier to understand

TODO

• Access to LightDM API from all windows
• Implement index.yml's nody-greeter configuration loader
• Implement cross window communication system
• Make web-greeter and nody-greeter compatible in terms of multi-monitor support (would require nody-greeter changes)
• Add secondary HTML files to gruvbox and dracula themes as nody-greeter currently offers.

[JezerM]

Finally, after a few long months of homework, I'm free to develop furthermore with this.

GreeterComm, a utility to ease communication between all windows, is finally implemented as greeter_comm in Javascript. This nomenclature change will be applied to nody-greeter soon.

Multi-monitor themes are not available yet.

[JezerM]

This should be done right now. It will be merged soon.