[Editor] Avoid "generating" the l10n-ids used for the a11yAlert

[Snuffleupagus]

In the DrawingEditor.prototype.#createDrawOutlines method the l10n-id was being "generated", which is bad for maintainability since searching for l10n-id becomes more difficult and the gulp check_l10n script actually warns about this.

Hence, similar to e.g. the resizer localization, let's define the a11yAlert l10n-ids once and provide shorthands for accessing them.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.17%. Comparing base (0c66063) to head (4072d3f).

Files with missing lines	Patch %	Lines
src/display/editor/editor.js	33.33%	2 Missing ⚠️
src/display/editor/draw.js	0.00%	1 Missing ⚠️
src/display/editor/freetext.js	0.00%	1 Missing ⚠️
src/display/editor/highlight.js	0.00%	1 Missing ⚠️
src/display/editor/stamp.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21263   +/-   ##
=======================================
  Coverage   56.17%   56.17%           
=======================================
  Files         220      220           
  Lines       59234    59236    +2     
=======================================
+ Hits        33273    33276    +3     
+ Misses      25961    25960    -1     

Flag	Coverage Δ
fonttest	8.64% <ø> (ø)
unittest	55.45% <14.28%> (+<0.01%)	⬆️
unittestcli	54.70% <14.28%> (-0.01%)	⬇️

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[timvandermeij]

Looks good to me; thanks!