ControllerEmu is a massive class with a lot of nested public classes.

The only reason these are nested is because the outer class acts as a
namespace. There's no reason to keep these classes nested just for that.

Keeping these classes nested makes it impossible to forward declare them, which leads to quite a few includes in other headers, making compilation take
longer.

This moves the source files to their own directory so classes can be
separated as necessary to their own source files, and be namespaced under the
ControllerEmu namespace.

Removed the unecessary forced tabbed layout, removed the layout part of the constructor and remade some method in preparation for tabbed styled input dialog such as the new hotkey configuration one.  It breaks every inputconfigDialog, but this will get fixed in the next commits.

Also moved to a folder since there will be many more files created in the next commits so it gives better separation.

For hotkeys, changed HotkeyManager to allow to get and make partial groups of hotkeys.

Also preserved the old configuration naming scheme for the ini, this is done to preserve compatibility with the older groups structure.

Add the ability to get GCPad control groups

Used like the HotkeyManager methods, this is used for the new GCPad configuration dialog.

Add the ability to get groups of Keyboard input

Same reasons as the previous ones.

Add ability to get groups of Wiimote input

Add the ability to get extensions group

This needed to pass to 3 classes.  Will be used for their respective dialogs.

This adds a recenter control binding which allows recentering the
cursor when relative input is enabled.

(EnableSettingControl is renamed to avoid confusions.)

This adds an option to enable relative input for the Wiimote IR
as described in issue 9014.

Enabling it will result in the pointer not going back to the centre
and the inputs will control the direction, not the absolute position.

Also adds a Dead Zone setting which is really needed when relative
input is enabled to prevent the cursor from slowly drifting on
most controllers. (Note: the Deadzone setting has no effect when
relative input is disabled)

This adds RemoveDevice() to ControllerInterface, fixes ExpressionParser
and some other code to support device removals without crashing,
and adds an IsValid() method to Device, to prepare for hotplugging.

The Setting class was used for both numeric values and booleans, and
other parts of the code had hacks to make it work with booleans.

By splitting Setting into NumericSetting and BooleanSetting, it is
clear which settings are numeric, and which are boolean, so there is
no need to guess by checking the default values or anything like that.
Also, booleans are stored as booleans in config files, instead of 1.0.