This fixes issues with Ctrl+T and the switch not reflecting correct
state when starting up -- previously, darkmode_switch was initialized in
construct_toolbar() as a local variable.

It wasn't clear what this button did without pushing it, at which point my note contents were uploaded to the Internet.

This particularly prevents the initial error all users will see when the
app first loads.

TODO: implement something more elegant.

Requires writeas-cli v1.1

Right now this mostly just includes (most importantly) undo/redo,
and less importantly smarter deletion and keyboard navigation.

Because changing the background colour of a textview for some reason
causes text selection not to show. This is a documented issue others
experience.

This code could be refactored a little now, but it works for the
elementary and HighContrast themes.

Because for some reason this would happen seemingly at random before.
No idea why. But I did test that it occurs during save, not read.

And it doesn't appear to happen anymore.

This is manually computed, because GTK is just complaining it doesn't support percentages.

Previously, the initial dark mode toggle would cause the default
settings to be saved, before font preferences ever loaded.

Closes #11

Moves the word count to the title bar subtitle.

This checks the returned error code from `writeas` and skips the browser
opening if the status != 0.

Turned out I was saving the theme before I had a chance to read
the values saved from the last run.

Though there seems to be problems reading it back out.

Now supports multiple fonts and the caller can be written more naturally.

TODO ensure it is installed with the packaging.

I slipped on this one -- the correct font value for monospace posts is
"wrap" not "mono". "mono" is more for code or terminal output, not the
writing that users will do with this app.