partially revert "remove "static fields" in FormMain."

This reverts commit 78db1ac9c1357294d4ded0dbf58af015fe06aa2d.

Conflicts:

	src/Cadencii.Application.Model/app_ui/models/main/FormMainModel.cs
	src/Cadencii.Application.WindowsForms/app_ui/forms/FormMainImpl.cs
	update_info/recent.xml

This change caused pianoroll editor unable to insert any events. Dunno why,
but it makes app useless. Hence reverting.