Note that this is a little bit unreliable with the keyboard shortcut
since LibGUI can get confused about which Action it's supposed to use
as each Browser::Tab has its own "close tab" action. This will need
to be fixed in LibGUI.
This patch moves most of the Browser UI into a Tab class. The main UI
now mainly consists of a GUI::TabWidget that Tab objects are added to.
I'm going with the "tabs on top" style here, since I like how it makes
it feel like each tab has its own UI controls (which it actually does!)
