[pcbnew] Preferences dialog event pile-up when moving (Windows-only)

Description

Unlike other dialogs in pcbnew, the Preferences dialog behaves differently when dragging it, causing the contents behind it to be redrawn for every intermediate cursor position, (and blocking further dialog events ?) to the point where the actual window position is lagging behind by several 100 ms. pcbnew_preferences_dialog_move_pileup

Steps to reproduce

• open and move the Preferences dialog

KiCad Version

Application: Pcbnew

Version: (5.99.0-2477-g0c774aa16-dirty), release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.66.0 OpenSSL/1.1.1d (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.1.1) nghttp2/1.39.2

Platform: Windows 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jul 30 2020 08:59:53
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.66.0
	ngspice: 31
	Compiler: GCC 9.2.0 with C++ ABI 1013

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=ON
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON