Consider adding polling for user-specified variables

[bblacey]

Currently the vera mqtt client only publishes variable values whenever a variable's value changes. This satisfies most use cases with an efficient propagation of data however in some cases, the dataset is too sparse for convenient analysis. It would be useful to allow users to specify variables that should be watched and/or polled at some periodicity. For example, polling TemperatureSensor1 or HumiditySensor1 or other variables would allow users to build a continuous dataset for certain sensors. In addition, a new companion mqtt client variable called mqttVariablePollingInterval should be added that allows users to specify the polling interval in seconds.

The existing user interface could be extended to include a second "polling" column that when checked, will add polling for the variable. Another option would be to add a second "Polling" tab.