Rewrite plugin command

The plugin_toggle and plugin_refresh commands have been rewritten into a
single "plugin" command. This takes commands such as "refresh" and
"toggle".

Also adds WebKit2 support where possible.
Disable the auth command in WebKit2

There doesn't seem to be a public API to do this as of 2.0.0.
Rewrite the cookie_* commands

Instead of cookie_add and cookie_delete, a new "cookie" command with
subcommands such as "add", "delete", and "clear" are added.

WebKit2 support is also implemented.
Disable unsupported content commands in WebKit2

More APIs missing.
Fixes for the spell_checker command
Implement cache and favicon commands for WebKit2
Reimplement search commands

Replace all search_* and the dehilight commands with a single "search"
command.

Also add support for options and WebKit2.
Implement security and dns commands for WebKit2
Clean up argument management

There were some memory leaks in various places because some args are
allocated on the heap and others are not. These functions ensure that
everything is cleaned up properly.
Rewrite js command

Instead of having a "js" and "script" command, have a single js command
with a "context" parameter. Also included is an uzbl context, separate
from the webpage. This should allow scripts and such to be written in JS
once some API functions are written.

Also adds WebKit2 support (but it's broken in that there doesn't seem to
be a way to get the JS context where the DOM and such live).
Add support for back/forward commands with WebKit2
Rewrite download support

The previous way of doing downloads required the callback function to be
exposed from gui.h. The new way fixes this because in WebKit2, a signal
gets sent for any download request. For WebKit1, a simpler API is
exposed in the gui.h header (without overloading the data pointer of the
callback).
Rewrite zoom_* commands

Instead of zoom_in and zoom_out commands, a single zoom command with
subcommands is implemented instead. The ability to set the zoom level to
a specific value is also added as well as one-shot zoom-step values.

Since WebKit2 does not have a zoom-step anywhere, it is simpler to do
the same for WebKit1 as well since the APIs are otherwise the same.
Implement hardcopy for WebKit2
Fix the snapshot command
Add attach/detach inspector subcommands in WebKit2
Simplify NULL check
Fix some style in inspector.c
Add WebKit2 support to inspector.c code
Add WebKit2 support for LINK_HOVER event