Bug 667862 - Allow mouse to optionally configure tablet devices

Add new setting GimpGuiConfig:devices-share-tool. When TRUE, we never
copy any properties between the user context and the GimpDeviceInfo's
context, so no tool or anything changes.

We do however still keep track of the active device so the setting can
be enabled/disabled at any time. Also hide GimpDeviceStatus' tool,
brush etc. indicators in "shared" mode.