Symbol editor: Create New Symbol button should probably be disabled if no library is selected

Description

When I click the "Create New Symbol (N)" button in the toolbar in Symbol Editor with no library selected, I get a banner saying "No symbol library selected".

From an UI perspective, it would probably be better to have buttons that are not supposed to work in the current state (i.e. when no symbol library is selected) to be disabled (greyed out).

Steps to reproduce

  1. Open symbol editor.
  2. Ensure that no symbol library is selected in the Libraries pane.

Expected behavior:

  1. "Create new symbol" button in toolbar is disabled and not clickable.

Observed behavior:

  1. "Create new symbol" button is active. Clicking on it produces a banner saying "No symbol library selected.".

KiCad Version

Application: KiCad Symbol Editor

Version: (6.99.0-1181-gc18d9b9baf), debug build

Libraries:
	wxWidgets 3.0.5
	FreeType 2.11.1
	libcurl/7.81.0 OpenSSL/1.1.1m zlib/1.2.11 brotli/1.0.9 zstd/1.5.2 libidn2/2.3.2 libpsl/0.21.1 (+libidn2/2.3.0) libssh2/1.10.0 nghttp2/1.47.0

Platform: Linux 5.16.11-arch1-1 x86_64, 64 bit, Little endian, wxGTK, Regolith:GNOME-Flashback:GNOME, x11

Build Info:
	Date: Mar  5 2022 11:51:59
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.78.0
	OCC: 7.5.3
	Curl: 7.81.0
	ngspice: 35
	Compiler: Clang 13.0.1 with C++ ABI 1002

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE_ADDRESS=OFF
	KICAD_SANITIZE_THREADS=OFF