[UI] Increment Control Values with Arrow Keys

Proposed Behavior
When an control's input box is selected and the user presses the up/down arrow key on their keyboard, the value increases/decreases by 1 unit.

Explanation
FF's UI seems to rely mainly on mouse input, but specifically with filter controls, I frequently find myself trying to increment values with the arrow keys (to no avail). The slider isn't always very precise, especially when the control features a large spread of values, and having to type in the desired values can be pretty tedious if you want to preview the filter for numerous consecutive values of a particular setting (6 --> [backspace] --> 7 --> [backspace] --> 8 --> [backspace] --> 9 ...). The ability to change input values using the arrow keys instead would be such a welcome change.