Added advanced page controls to inventory

In response to this issue:
https://github.com/jshackles/Enhanced_Steam/issues/273

Added button to go to first page, last page, and to specific page.
Here is what it looks like: ![](http://i.imgur.com/rKpo3yT.png)

There's a few potential tweaks that would be tricky to do.
1. Currently, first/last button don't get disabled if you are on the first/last page. To do this, you need to actually hook into the functions for NextPage and PreviousPage and it'd become much more of a hack job than it currently is.

2. The input currently has no min/max bounds, might want to add that. Then again, Firefox doesn't even support type="number" apparently? I do bound checks when changing the page though, so it's less important I guess.

3. I've experimented with various character for the first last button, such as « » or ≪ ≫. The problem is that they look so different from the original prev/next buttons, but we could change those too. I'll let you guys decide which looks best.