Altvstime tool implemented with QCustomPlot library - Compute Altitude by Time Button

As my previous commit presents, now is available a very easy way to get the information from the curve. All the user needs to do is to right-click on a point on the curve and a tooltip ( information box ) will show up.
In this commit I added the Local Sidereal Time ( LST ) in the tooltip. It contains now the Local Sidereal Time ( LST ), Local Time ( LT ) and Altitude.
I also implemented an useful way to get the Altitude for a certain moment of time. You need to select the hour and minutes and then press the "Compute" button. The skyobject can be selected directly from the PlotList. This brings more flexibility to Altvstime tool.

I attach here a short videoclip for a better understanding:
https://www.youtube.com/watch?v=25P9tPxJECs

CCMAIL: kstars-devel@kde.org