Previously, "Serif" would be selected in the menu when the app first
loaded, no matter what the actual setting was.

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.

Screenshots here and on https://write.as were taken on elementaryOS
Juno.

This closes #12

This closes #11

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.

This reverts commit 4c784b32c8.

This reverts commit c3b0a35a2e.

Opening a text file would overwrite the current draft, so for now we'll
disable this.

- Mention User Guide
- Mention CLI dependence
- Improve meson install instructions

Requires writeas-cli v1.1