* feat: add TargetPlatform `isDesktop` helper
Safe and convenient way to check if the current platform is a desktop platform.
* refactor: move window logic to separate class
This is much cleaner and better encapsulated. Sets up an area for
further window management in the future.
Does not try to initialize on mobile or web, which would crash.
* fix: isDesktop doc comment
* fix: set window title
Currently only displays "app_flowy" as the window title. This commit
sets the window title to "AppFlowy" as part of the init process.
