xres: add key bindings for scrolling half the page
xres: add key binding for fullscreen toggle
vim: set guifont; add support for font resize

 * set default font for gvim
 * add function that changes the size of the font based on given amount
 * add shift+keypad_plus and shift+keypad_minus mappings for making the
   font larger or smaller
vim: add mapping for the default font size

add shift+keypad_multiply mapping to change the font size back to the default
xres: add key binding for the default font size

add shift+keypad_multiply binding to change the font size back to the default