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.
Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).
