Remove all GTK2 and WebKit1 code.
Handle fullscreen mode; add keybinding to toggle fullscreen.
Don't forcibly resize all windows on initial startup.
Don't show any windows until all startup scripts have run.

This fixes the annoyance where the initial window would resize itself *after*
becoming visible if wkb.default-width or wkb.default-height were set in the
startup scripts.
Handle close signal; minor code cleanup.
Add command to remove user stylesheets.

'add-stylesheet' command has been changed to 'add-ss' as well.
Allow building with webkit2gtk-4.0 (v2.5.1 and greater).
Update code for GTK 3.14.
wkb.c: Make update_title() work properly with WebKit 2.6.1.
Update cb_decide_policy() for WebKit 2.6.0.