Shift + scroll, left/right arrow keys to scroll horizontally

[dshepsis]

In some cases, there can be a horizontal scrollbar in the left sidebar or in the main window. Currently, the only way to scroll horizontally is to click and drag the bottom scrollbar. For ease-of-use, please allow holding shift while scrolling with the mouse to scroll horizontally instead of vertically. Additionally, please allow the left/right arrow keys to scroll horizontally when they wouldn't have another effect (e,g, collapsing/expanding a pointer).