Reimplement the code to modular design

[nedik]

With current design it's hard to switch to other libraries so we want to have modular design. And then we could switch for example from wxWidgets to Qt easily in the future.

By modular design i mean having functionality separated into modules so when one module is deleted the whole program still works somehow and it's not that devastating.

Example structure would look like this:

1. Frontend (all drawing, wxWidgets currently)
2. Backend (functionality of tools and others)
3. Core (communication between modules)