Add HEX editor

[martinrotter]

How it will work:

• inherit Tab class,
• custom widget displaying "hex" representation (based on Scintilla) on the left,
• TextEditor on the right,
• will have its own plugin menu which will allow to open file and display it in hex editor,
• only "visible" part of file is loaded (?),
• text editor displays only ASCII text and all other data bytes (including for example newlines) are displayed as dots, for example when we have utf-8 text "člověk" then text editor on the right will show "..lov..k"

https://github.com/virinext/QHexView