Fixes https://github.com/vector-im/riot-web/issues/8936
Watchers are now managed by the SettingsStore itself through a global/default watch manager. As per the included documentation, the watch manager dispatches updates to callbacks which are redirected by the SettingsStore for consumer safety.
This implements a dream of one day being able to listen for changes in a settings to react to them, regardless of which device actually changed the setting. The use case for this kind of thing is extremely limited, but when it is needed it should be more than powerful enough.
Fixes https://github.com/vector-im/riot-web/issues/6435
This is done through an on-the-fly inverter for the settings. All the settings changed are boolean values, so this should be more than safe to just let happen throughout the SettingsStore. Typically a change like this would be done in the individual handlers (similar to how setting names are remapped to different properties or even different storage locations on the fly), however doing that for this many settings would be a huge nightmare and involve changing *all* the layers. By putting a global "invert this" flag on the setting, we can get away with doing the inversion as the last possible step during a read (or write).
To speed up calculations of the default values, we cache all the inverted values into a lookup table similar to how we represent the defaults already. Without this, the DefaultHandler would need to iterate the setting list and invert the values, slowing things down over time. We invert the value up front so we can keep the generic inversion logic without checking the level ahead of time. It is fully intended that a default value represents the new setting name, not the legacy name.
This commit also includes a debugger for settings because it was hard to visualize what the SettingsStore was doing during development. Some added information is included as it may be helpful for when someone has a problem with their settings and we need to debug it. Typically the debugger would be run in conjunction with `mxSendRageshake`: `mxSettingsStore.debugSetting('showJoinLeaves') && mxSendRageshake('Debugging showJoinLeaves setting')`.
