Moved some settings from gvimrc into vimrc

Some keybindings weren't working because sourcing mswin.vim overrides
them.  Having gui keybindings in vimrc lets us override some things. The
only thing left in gvimrc are gui options & window size.
Have <leader>w close window instead of destroy the buffer
Set <leader>b to use ctrl-p buffer list
Add C-g mapping in command mode (muscle memory from evil-mode)