Allow interacting with the tab line from a touch screen

Allow interacting with the tab line from a touch screen

* doc/emacs/frames.texi (Tab Bars): Explain how to interact with
the tab bar from a touch screen.
* doc/emacs/input.texi (Touchscreens): Document exactly what a
``long press'' is.
* doc/emacs/windows.texi (Tab Line): Likewise.
* lisp/tab-line.el (tab-line-tab-map, tab-line-add-map)
(tab-line-tab-close-map, tab-line-left-map, tab-line-right-map):
Bind `touchscreen-begin' to each command.
(tab-line-track-tap, tab-line-event-start): New functions.
(tab-line-hscroll-right, tab-line-hscroll-left, tab-line-new-tab)
(tab-line-select-tab, tab-line-close-tab): Use them.