kicad: No error is thrown if opening multiple instances of the same project

Description

In case the same pcb or sch is tried to open with standalone pcbnew or eeschem - erro is thrown "File is already opened". The same does not currently apply to kicad_pro files, but it should.

Steps to reproduce

1. Open two instances of kicad;
2. Open the same kicad_pro file;
3. No warning is thrown;
4. Both instances can keep the same project opened -> possible data corruption?

KiCad Version

Application: KiCad

Version: (5.99.0-9996-g3279e295ac), release build

Libraries:
	wxWidgets 3.1.4
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19042), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Mar 25 2021 13:06:53
	wxWidgets: 3.1.4 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1928 without C++ ABI

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