Fixed issues with existing sample scripts

[romainl]

• messages: removed opening of noisy README.md
• quickfix: updated the grep input
• popupmenu: made the buffer a scratch buffer and shortened the menu

[romainl]

I will split the bloated sample_base.vim into two or three more manageable ones.

--- EDIT ---

Done:

• sample_terminal.vim

  

• sample_misc.vim

  

• sample_windows.vim