Design issues in preferences window

[kasperfm]

I'm using Boostnote v0.8.17 on Ubuntu 17.10 64bit, and found two issues.

• Strange black (check)box at the top of the preferences window.
• "Info: Release Note: v0.8.17" is too large for the sidebar.

[kohei-takata]

@kasperfm I will close this issue, because this repository is for boostnote-mobile. Could you open this issue in this repository?