UI|Control Panel|Client: Added Input Settings, removed old "Input" page

A new config variable (Config.input.mouse.syncSensitivity) controls
whether the X and Y axis sensitivities are always the same when
modifying them in Input Settings.

Added an option to span cells in GridLayout, and sliders use double
values for extra precision. Sliders also support applying a factor
to the displayed values (does not affect the actual values).