EFocusTracker tracks the input focus within a window and helps keep
the sensitivity of "selectable" actions in the main menu up-to-date.
Selectable actions include Cut, Copy, Paste, Select All and Delete.

EFocusTracker has built-in support for widgets that implement the
GtkEditable interface such as GtkEntry and GtkTextView.  It also
supports custom widgets that implement the ESelectable interface,
which is a subset of GtkEditable and can apply to anything that
displays selectable content (esp. tree views and ETables).

This commit integrates EFocusTracker with EShellWindow, CompEditor,
EMsgComposer, and ESignatureManager.

It also bumps the GtkHTML requirement to 2.29.5 to utilize the new
GtkhtmlEditor:html constructor property.