Hardcode maximum supported probes to 64 for now

SR_MAX_NUM_PROBES disappeared from libsigrok.h, along with that
limitation.
Added missing virtual destructors

This fixes bug #106.
Fix "Failed to configure samplerate." warnings.

We need to keep the sample rate selectors hidden until they have been
reconfigured and we need to ignore the samplerate when the device
selector is hidden.

This fixes bug #109.
Added DeviceManager

This class now manages the application device list, scanning for
devices, and releasing them when they are no longer needed.