This is a workaround for GCC bug #56480, #78274, present in gcc 4.8 -- 7.0.

Can implement different drawing, depending from active part of the
panel. 

Search for object in sub-sub pads before creating new TCanvas.
Drawing copy of function - do not keep stale pointer as was done by
previous panel

Object from different locations (gDirectory, panel list) grouped in
ComboBox. 

Also functions list adds previousely used fits. They correctly updated
when selected object is changed. 

Support different kinds of objects, prepare grouping

Now models can be grouped by different categories
Fir function for the fitting stored in extra map and can be selected in
the future

Start making code comparable with TFitEditor.
Major methods will be copied step-by-step to RFitPanel - with some
adjustments

Provide more generic names, while can be used in many different widgets. 
In the future that small class can be move to some more generic library

Add missing boolean flags, process them when generate fit options

Instead of creating multiple lists and switching between them use ui5
Filter to preselect items for ComboBox. Need some programming - not
possible pure XML solution

As a side-effect, this change solves the install problem with a missing
tree/forest/inc directory. Fixes ROOT-10123.

A second try to appease the incremental bots.

use bold instead.

since it does not exist in the c++17 stl.

Name ("RForest") still TBD!