Adjust GVim window size

Since I tend to use a line width around 80 in most files, it doesn't
make sense making Vim wider than that. By default it's now narrower,
more portrait-like, instead of filling up so much of my screen with
white space.

In future it might be nice to add an autocommand or something that makes
GVim wider when I open a diff or a vertical split.
Hide menubar / toolbar / scrollbar in GVim by default

A while ago I added keyboard shortcuts for hiding these graphical "bars"
because I so infrequently use them, but after a while I noticed a
pattern: every time I open GVim, the first thing I do is press
<C-F1><C-F2><C-F3> to hide them all and save space on the screen.

Now these are hidden by default and can be shown when needed.