Implement notification updates for footprint/symbol libraries (lp:#1831755)

[Original report](https://bugs.launchpad.net/bugs/1831755) created by **Piotr Gałka (piotrgalka)**
```
I have moved (using file management programm) some footprints betwean
libraries and now the footprint lists showed by Footprint Editor disagree
with files list in directories.
I include screen shots of directory lists and Footprint editor window.

I just don't believe in what is going on. Noticing the problem I have
switched the computer off and on to force KiCad to reread directories. I
have done it twice but with no effect.
I just supposed that KiCad reads the footprint lists from directories each
time I run Footprint editor.

But now during writeing here I should say something - at second computer I
have also the same KiCad version installed but not with my libarary list,
but with ortiginal lists. When I run Footprint editor at that computer it
behaves diferently from day to day. Sometimes it looks that it hangs during
importing footprints at the beginning, but after more than one minute it
finishes and looks OK. Sometimes it takes about 30 seconds to run Footprint
Editor and sometims it looks like it needs no time - just run, and in one
or two seconds it is ready.

Now come back to my working computer (it is not connected to internet so
Windows 7 is not updated since 2 years, but I would not suppose it would be
a source of problem).

Today I decided to move all my PLxxyy.kicad_mod from my S library to my H
library.
I started doing it with KiCad using 'Save as' and 'Delete' - this way I
moved PL0512, PL0513, PL0613. Then decided to move rest faster using file
management program (FreeCommander). I have done it (probably not closing
Footprint Editor, but not sure). And now the situation is as you can see in
attached screenshots.
When I try to enter (double click) the unexisting footprint - just nothing
happenes.
I have reopened Footprint Editor, and whole KiCad several times, I have
swithced off and on computer - no effect.
I tried to find "PL0815" text in my KiCad installation directory and in my
AppData/Roaming/kicad directory to discover where from KiCad can get this
old directory situation. I didn't found.
To be sure I am not cheat by FreeCommander I looked at directory contenets
using Windows Explorer - all files are where I think they are.
The most I am surprised is - where from KiCad could get libabry lists it
showes.
I just don't believe in what is going on.

I plan to move files back, and then move them using KiCad and hope problem
will gone. But certainly something is going wrong.
I have copied kicad_mod files from KiCad libararies to my libraraies many
times duirng last month with absolutelly no problem. After reopening
Footprint Editor the footprint list was updated.

Application: kicad
Version: (5.1.2)-1, release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.61.1 OpenSSL/1.1.1 (WinSSL) zlib/1.2.11 brotli/1.0.6
libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.5) nghttp2/1.34.0
Platform: Windows 7 (build 7601, Service Pack 1), 64-bit edition, 64 bit,
Little endian, wxMSW
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
    Boost: 1.68.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.61.1
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    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_USE_OCC=OFF
    KICAD_SPICE=ON
```
Original tags: eeschema pcbnew

Story points: 6.0