Fixes compatibility with Thunderbird 68.

Changelog:

Add match for dialogs like: %1$S is requesting your username and password. WARNING: Your password will not be sent to the website you are currently visiting!

This is the initial release with Thunderbird 60 support. Big thanks to @Skywalker-11 for doing most of the work here.

There is one important change to be aware of. The way you access the preferences dialog has changed. There is no longer a button in the Add-ons Manager. Instead, use the "hamburger" menu:

<img src="https://user-images.githubusercontent.com/963645/44045266-995becfc-9eed-11e8-8317-084846701658.png>/>

... or the application menu *Tools* > *Add-on Preferences* > *KeeBird*:

<img src="https://user-images.githubusercontent.com/963645/44045317-c80b8fda-9eed-11e8-8923-e3dc050e0b0d.png"/>

Fix main window resizing

• Remove residual metrics option strings
• Fix relative paths for KeePass.exe not working on Windows