Bug fix related with the shortcuts service

Updates in the keyboard interaction in the search

* Fixed a bug in that made some HTML tags being added in wrong places
* Updated the search UI and its behavior

Updated how styles are added

Removed one CSS style that made in some cases (not in Firefox) text fields not to show their content.

Now resizing the panel is also compatible with keyboard. The resizer element can get the focus. Once it has it, using the arrow keys the user can change the width of the panel.

Improved the way in which the panel is shown. Now it tries to identify fixed elements in the page and to adjust their position, so they are not covered (and hidden behind) the panel.

The latest changes include:
* Fixed the minimum font-size, so if the page sets a root fontsize smaller than the default one from the browser, the extension still keeps the browser default as minimum.
* Fixed the style in the document selector (shown when there are frames in the page) of the tab HTML 5 Outline.
* Other minor fixes.

* New language added: Japanese
* Fixed the bug that made the document selector appear twice in some cases
* Fix the way in which the dialogs were loaded, so the language change happens when closing/opening the extension instead of when reloading the page

Improved the way in which the extension identifies whether a header is hidden or not, so now it considers the "details" HTML element and also whether iframes are visible or not

* When there are hidden iframes, they are not considered in the document selector in the HTML5Outline tab unless they option to "show hidden sections" is active.
* Improved styling of the icon position that identifies hidden elements.

Bug fix that prevented the proper behavior of the extension

* Improved the way in which the labels are loaded.
* Reduced the extension files requests.
* Keep the selected document in the dropdown when changing between analysis.
* Improved dialog dimension estimation.
* Minor fixes.

Bug fix in the search feature

Improved performance
Bug fixing

Improved keyboard interaction with the tree

Shortcuts improvement and minor fixes

* New keyboard shortcuts to interact with the extension. Now it includes some shortcuts to trigger actions like the search function, collapse nodes in the results, open the settings dialog, and even open the dialog to show the shortcuts. Since using shortcuts could collide with some shortcuts from assistive technology, I made them configurable. Some have a default value and others are not defined yet, so users can configure them based on their needs.
* The buttons to change the panel position and access to the settings are now part of the main menu. The main goal was to simplify the view and also to reduce the keystrokes when interacting with keyboard.
* Option to hide tools that the headings tab offers (buttons to collapse by levels, search and copy buttons. The main goal, as already mentioned, was to simplify the view. Although the search button is hidden, the shortcut key for the search still works and shows it. It is just visually hidden and only available with keyboard.
* Accessibility improvements.
* Improvements in the behavior of the dialog, menus, etc.

Improved accessibility and styles

Fix styles error

Update in the keyboard interaction for the buttons to collapse/expand by level

* New search feature to find headers in the tree.
* Improvements in the buttons to collapse/expand the tree by levels (improved accessibility, behavior, keyboard interaction, etc.).
* Changed the trees to use the proper ARIA role and expected keyboard behavior.
* Improved button sizes.

Fixed a bug that prevented showing results when there were img elements without src attribute

Updated the style of the resize bar so now the cursor shows the whole bar as interaction element.
Fixed a bug with the "collapser" buttons shown when missing intermediate heading levels.