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.
Add a GimpDeviceManager object and take over most code from
gimpdevices.c, but leave all functions in gimpdevices.c there as
wrappers in order to make rebasing in gtk3-port simpler.