First pass at selectable toolbar button styles

There’s not much to making the toolbar button styles switchable: the
change to the PageTemplate introduces a new configuration tiddler that
controls the button styling. Most of this commit is the user interface
for changing that setting, including the translatable text. I think
this again demonstrates that we need to be very selective about which
configuration options have a UI included in the core. Otherwise, a few
dozen more of these settings will start to become a significant
fraction of the core.

core/ui/ControlPanel/Settings/ButtonStyles/Boxed.tid

@@ -0,0 +1,5 @@
+title: $:/core/buttonstyles/Boxed
+tags: $:/tags/ToolbarButtonStyle
+caption: {{$:/language/ControlPanel/Settings/ToolbarButtonStyle/Styles/Boxed}}
+
+tc-btn-boxed