[FEATURE] - input & output gain fields: allow typing numerical values with keyboard

[mstgrv]

At the moment, input & output gain controls are sliders, but you can't enter numerical values with the keyboard. Would be a big improvement to have this ability.

[jatinchowdhury18]

Thanks for reporting this issue. At the moment, when you click on the bottom bar sliders, the UI will wait 270 milliseconds, and if a second click has not occured then slider should become a type-in prompt. The reason for the 270 millisecond wait is in case the user is double-clicking the slider, in which case the slider will be reset to the default value.

If the type-in prompt is not showing after the wait, then there's a bug happening somewhere, which we'll need to fix.

If it seemed like the type-in prompt was not showing because the wait was too long, then maybe we need to make the wait time shorter? I'll have to play around with that a little, but I imagine there might be some variety with different mouses, or especially with touch screens.