Eeschema: Unify the state of the choose symbol after opening and when the component is not selected

Description

Unify the state of the choose symbol after opening and when the component is not selected.

It would be more user-friendly if the background color used in the eeschema was used for the preview instead of white.

The text color "No symbol selected" could also be used with the colors defined in the appearance of the eeschema, for example for labels or texts.

Steps to reproduce

vokoscreen-2020-07-28_00-56-11

KiCad Version

Application: Eeschema

Version: 5.99.0-unknown-8324cb3~102~ubuntu18.04.1, release build

Libraries:
	wxWidgets 3.0.4

Platform: Linux 5.3.0-62-generic x86_64, 64 bit, Little endian, wxGTK

	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
	Boost: 1.65.1
	OCE: 6.9.1
	Curl: 7.58.0
	ngspice: 32+
	Compiler: GCC 7.5.0 with C++ ABI 1011

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
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON