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)
dont use split() but os.path.splitext()
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.
just write musicxml output to file

todo: use the write() method of the elementtree document, this also
handles encoding correctly (now it is encoded twice in some cases possibly)
neater to just use XML files and not other types
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
highlight-timing: Restore one-second default

I think this update should be enough to restore the default
behaviour from existing Frescobaldi.

IISC AbstractMatcher is only used in this context, so I can simply
hard-code 1 second here.
Merge pull request #254 from uliska/highlight-timing

highlight-timing: Restore one-second default
changed to xml.etree
Merge pull request #256 from PeterBjuhr/export-musicxml

changed to xml.etree
Add HtmlPage class WIP