[a11y] inoMarkdownEditor: Missing aria-lables for icon buttons

[Sl1nd]

For Icon Button navigation no 'aria-labels' are provided. As of this screensreader users may not use Markdown Editor. To support i18n optionally configuring labels needs to be provided via properties.

Expected behavior

Add descriptive 'aria-labels' to icon-buttons e.g.:

           <button
             aria-label="Edit text"
              class={getViewModeBtnClasses(ViewMode.PREVIEW)}
              onClick={() => this.handleViewModeBtnClick(ViewMode.PREVIEW)}
            >
              <ino-icon aria-label="Edit" icon="edit_text" />
            </button>

Propably a default language should be used with the option to configure 'aria-labels' with consumer language.