Structural updates

[HenryQuan]

• Wrap all globals in a class (App)
• Move out logic from UI (mainly render methods)
• Cache data better (download automatically if needed)
• Add local cached data #57

[HenryQuan]

Should we update the structure or simply write it Flutter? The flutter app is half done. The only left is localisation, but we can use existing ones. Does it worth the update?

[HenryQuan]

We will be using Flutter for the final rewrite. This discussion will be moved to a new issue.