Wishlist: Offer to update lib tables (lp:#1820305)

[Original report](https://bugs.launchpad.net/bugs/1820305) created by **Evan Shultz (evan-shultz)**
```
As a librarian, I'm bouncing between different library PRs all the time
during reviews which may have branched before new libraries were added or
removed. Perhaps the below concept doesn't affect the general KiCad users,
or affects them less than me, so keep that in mind.

When the [symbol|footprint] library table contains a library folder at the
library path (env var) that doesn't exist, there is an error message dialog
when opening libedit or eeschema. And there is a dialog for every single
error. The dialog allows me to not see an error again, but since libraries
appear and disappear as I switch branches I never check that box. I always
keep my library tables in sync with the head rev of the library master and
then deal with error that come about. Seeing this dialog frequently is
annoying because it's not something I will actually solve since it goes
away once I merge the PR branch into master. Pcbnew and the footprint
editor have a similar dialog, again once for each missing library folder,
but there is no checkbox.

Conversely, if there are libraries at the library path but not in the
library table there is no alert about that and libraries can be missed.
This could also apply to symbol or footprint libraries.

I wonder if there is a way to improve the situation when the library table
and library folders have a mismatch?
1. If the library folder is missing. If I check the box in the error dialog
I might miss something later on when I switch to the master branch. I guess
there is a way to disable to "never show again" checkbox but I don't find
it user-accessible. Is it? If not, perhaps put this option in the tool UI
since someone could regret being able to turn on "never do something" and
not turn it off? It might also be nice to load all libraries and then show
a single dialog of all missing library folders instead of individual
dialogs.
2. If there are extra library folders. The potential solution here is more
obvious and straightforward. A dialog could appear which offers to update
the lib table and add the new libraries. And again, a single dialog for all
new libraries would be nice. And a "never show again" checkbox on the
dialog, again with a re-enable in the tool options, would work.

I understand if this is too much of a niche situation to spend any time on.

Application: kicad
Version: (5.1.0)-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 8 (build 9200), 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
```

Story points: 6.0