icons: Add text-x-generic

The file chooser needs at least one icon that we can
fall back to for non-directories. text-x-generic is
that icon.

gtk/icons/scalable/mimetypes/text-x-generic-symbolic.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16.014" height="16.014"><g fill="#474747" fill-rule="evenodd"><path d="M3.014 1.014a1 1 0 00-1 1v13a1 1 0 001 1h10a1 1 0 001-1v-9.5a1 1 0 00-.293-.707l-3.5-3.5a1 1 0 00-.707-.293zm1 2H9.1l2.914 2.914v8.086h-8z" style="line-height:normal;font-variant-ligatures:normal;font-variant-position:normal;font-variant-caps:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-feature-settings:normal;text-indent:0;text-align:start;text-decoration-line:none;text-decoration-style:solid;text-decoration-color:#000;text-transform:none;text-orientation:mixed;shape-padding:0;isolation:auto;mix-blend-mode:normal" color="#000" font-weight="400" font-family="sans-serif" overflow="visible"/><path d="M9.014 2.014v4h4z"/></g></svg>