Add an All problem list

[twchn]

🚀 Feature Proposal

Add a new problem list which contains all leetcode problems.

Motivation

Maybe someone such as me wants to solve problems in order.

[yucaowang]

+1

[Vigilans]

@jdneo I think the All Problems part in #188 could be extracted and added to 0.13.0 milestone. It does not involve the complex logic of updating tree view.

[jdneo]

@Vigilans I'm thinking to refactor the explorer before adding new features to it. For example, extract the logic of node reference to a cache called something like xxxCache or nodeManager...

Then the explorer will only be used as a controller