uisupport: Allow to disable spell checking

Apparently, Sonnet, when used standalone (i.e. not in KTextEdit),
does not honor its own setting to disable it by default.

Explicitly read the setting when initializing the highlighter, and
set its active state accordingly. Provide a context menu option
to enable/disable spell check on-the-fly as well.

Remove the bogus/unused checkbox in the InputWidget settings page;
there is a dedicated settings page for configuring spell checking.

Explicitly depend on Sonnet when building with KDE Frameworks
(it was transitively depended upon anyway), so we can be sure to
offer the Sonnet settings page even when using Frameworks.