<perspective-workspace> toolbar fix

[texodus]

• Fixes the theme selector UI to have a fixed max width (to prevent excessively long theme na,es from making this control break layout).
• Fixes perspective-config-update event for <perspective-viewer> to correctly fire when the title config property has updated (via keyboard onchange, which only fires on blur).
• Changes the theme selector behavior to stay in "open" mode as long as the control has focus, which makes experimenting with different themes with the keyboard easier.
• Added a new keyboard navigation mode to the theme selector which allows up/down arrow + shift key to quickly switch between themes.
• Fix perspective toolbar button sizes when embedded in a <perspective-workspace>, which overflowed a few pixels previously.
• Added tests to <perspective-workspace> to validate that layouts which remove or re-use master/detail workspace configs restore as expected.