Added persistent default country and switch between global and country

[Tushar-OP]

#7

it would be nice to add the ability to set a persistent default country, which perhaps also loads the charts by default.

• Added the "Default to country view" in settingsDialog.dart
• Now, user can select their country and it will show that country's stats if the "Default to country view" is true.

if a default country is chosen, a simple button in the top bar to quickly switch between the default country and the global stats may be interesting

• They can do that by toggling "Default to country view" in settings