Strobo preview improvements

[mcallegari]

I'm opening this as a reminder, for who wants to pick up this task.
Sooner or later this has to be done anyway, cause I want to have a reliable preview in QLC+ 5.

The changes have to start from the ground up. The involved components are:

1. Fixture Editor: a Shutter channel should expose the following capabilities:

• shutter open
• shutter close
• specific frequency
• frequency range
  If the frequencies are not specified in the fixture manual, and instead there is something like "Strobe (slow to fast)" I would say the default behaviour should be an implicit range of 1Hz to 10Hz

2. QLCCapability class: likewise the color/resource properties we could add:

• shutterType: Enum { Close, Open, Fixed, Range }
• shutterMinFreq
• shutterMaxFreq
  In case of Open/Close the Freq variables will be ignored
  In case of Fixed, shutterMinFreq will be used and shutterMaxFreq will be ignored

3. 2D Monitor: when we have all the information we need in a QLCCapability, then the monitor will be easy to implement, as long as every FixtureItem has a QTimer to handle the strobe frequency

[janosvitok]

3. partially fixed: cf58886 (added strobing timer using currently available info)

[janosvitok]

See also http://www.qlcplus.org/forum/viewtopic.php?p=45107

[mcallegari]

This can finally be closed. Addressed with #1026