Override shortcuts in HTML input fields

When users defined a single-letter short cut it was not possible
to type this letter in HTML input fields.

Fix this by accepting ShortcutOverride events whenever the web page
is editing text.

Use QInputControl::isCommonTextEditShortcut for Qt 5.9 and later.
For the case where QtWebEngine is built against an older Qt a duplicated
code path is used.

Also, ensure users do not override web action short cuts.

Task-number: QTBUG-59053
Change-Id: Ic26cf2a040a72b118273c6645c00b2913b995b0b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>