Enhanced conditions for conditional styles (LED, Attribute Display)

In certain widgets (at least LED and Attribute display) we need to extend the expressivity of conditions that allow selection of a given CSS style. What is needed is the ability to express that:

• the current value V is (where k,k1,k2 are constants, v1 and v2 are current values of OTHER attributes of the device):
• V </<= k
• V>/>=k
• V </<= v1
• V>/>=v1
• k1 </<= V </<= k2
• v1 </<= V </<= v2
• v1 </<= V </<= k
• k </<= V </<= v1

 An example where the v1 < V < v2 kind of relation is needed is a power monitor device, where the thresholds on the attribute are defined as values provided by a second attribute. This is a way to create dynamic thresholds.

Constants should be the ones specified as metadata of the device attribute, namely min, max, warning and alarm. In this way what the user would see is what is defined in tango devices, not another representation stored in the dashboard.

To an LED or Attribute Display widget there can either be the current comparison (of its value against a literal numeric constant entered by the user) or an "Advanced comparison"  that expresses any of the 8 boolean conditions mentioned above. They should be mutually exclusive and exhaustive but Taranta however would not check either of these properties.
The "Advanced comparison" is a toggeable feature (enabled for SKA and disabled for MaxIV, by default).