.kicad_dru files need their own icon

Description

Currently, the .kicad_dru files show up in the project tree with the '???' file icon. We should create a specific icon for these files or remove them from the project tree view.

KiCad Version

Application: Eeschema

Version: (5.99.0-8689-g945faba586-dirty), release build

Libraries:
	wxWidgets 3.0.5

Platform: Linux 5.8.0-3-amd64 x86_64, 64 bit, Little endian, wxGTK, xfce, x11

	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.5.0
	Curl: 7.74.0
	ngspice: 33
	Compiler: GCC 10.2.1 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON