allow to override treeview fontsize

[devel0]

this PR allow override treeview fontsize using an environment variable ILSPY_TREEVIEW_FONTSIZE.

Example:

ILSPY_TREEVIEW_FONTSIZE=14 ILSpy

[jeffreye]

The proper way to configure font size is to modify the configuration file. We would like to keep it the same way as ILSPY.
If you want to make it a new feature, please create an issue at https://github.com/icsharpcode/ILSpy. We can port the implementation from ILSpy to here later.

[p1gp1g]

Thanks @devel0. It is pretty unusable with Wayland without this (the X11 scale factor doesn't work).