highlight-timing: Make highlight matches infinite

Initial idea: Don't stop the highlighting of matching brackets after
one second.
This will of course have to be made configurable later.
highlight-timing: load highlight time from settings

Loads the value for the highlight time from the settings.
Currently there isn't a user interface for this so this means it will
always be 0 (= infinite)
highlight-timing: create Preferences page (init)

Create a new page for the Preferences dialog "Editor Options".
This is for now simply a copy of the "Helpers" page.
highlight-timing: Make Preference page work (init)

Now the Preferences page works for highlighting of matching brackets,
the value set here is respected in the matcher.
highlight-timing: Only load settings when changed

The value for the highlighting duration is now read from QSettings
only on app.settingsChanged (instead of on each match)
highlight-timing: Use theme icon

I'm not sure if that's a good one, though
Merge pull request #253 from uliska/highlight-timing

Make match timer configurable