* Added mouse forward/back button support on Linux
* Added browser history navigation action bindings for Windows, Mac, and Linux keyboard
* Removed new `history-action` event and modified `app-command` to execute on such events
* Removed attempt at macOS support and added constants
* Clarified app-command documentation
* Reverted 'app-command' description change
* Format code
* build: define compile time features with buildflag header
* refactor: switch to BUILDFLAG(ENABLE_DESKTOP_CAPTURER)
* refactor: switch to BUILDFLAG(ENABLE_RUN_AS_NODE)
* refactor: switch to BUILDFLAG(ENABLE_OSR)
* refactor: switch to BUILDFLAG(ENABLE_VIEW_API)
* refactor: switch to BUILDFLAG(ENABLE_PEPPER_FLASH)
* refactor: switch to BUILDFLAG(OVERRIDE_LOCATION_PROVIDER)
* refactor: switch to BUILDFLAG(ENABLE_PDF_VIEWER)