Add Page-up/down scroll functionality in inventory/deck.

[Asheram]

Is your feature request related to a problem? Please describe.
Currently it's cumbersome to use the mouse wheel to scroll through the inventory, especially as the inventory increases to contain thousands of cards.

Describe the solution you'd like
While it has been suggested that a scroll tab would be difficult to implement, I suggest instead that you could use the Page-Up/Down keys to scroll through the inventory.

Describe alternatives you've considered
An alternative would be if you could select a card and use the basic arrow keys to navigate through the inventory.