Horizontal scrolling and drag/drop tab selection

Two changes would make the tabbed interfaces of the game more accessible to players.

- Make use of the horizontal scrolling functions of modern mouses. Have the left and right scroll functions (currently registered in the game as Mouse3 and Mouse4) switch between tabs when a tabbed window is selected.

- When drag-and-drop functionality is active, for instance when a player is dragging an inventory item, switch tabs in the window automatically when the cursor is hovering above the tab. This is how things work in most tabbed interfaces. Right now, the game just highlights the tab name the same as if you were not in drag-and-drop mode and its kind of a bit of a tease.