Added some basic showRegister handliing.

ExecuteHandler calls Vim.showRegisters, which updates Vim's console
property. After the console property has changed, the Editor view picks up
the change. Does not yet put any markup on the page.
Render text to Vim's console.

:reg command triggers update of Vim's console property.
Renamed EXECUTE to CMDLINE.

To be more in line with Vim documentation.
Renamed files.
More renaming of stuff.
Small hack to fix :reg handling.

CmdlineHandler has a flag, isInShowRegisterMode which controls how the keys
received immediately after a :reg command are handled. Any key pressed will
close the register listing and go back to NORMAL.