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/ToolbarButtonStyle.tid

@@ -0,0 +1,12 @@
+title: $:/core/ui/ControlPanel/Settings/ToolbarButtonStyle
+tags: $:/tags/ControlPanel/Settings
+caption: {{$:/language/ControlPanel/Settings/ToolbarButtonStyle/Caption}}
+
+\define lingo-base() $:/language/ControlPanel/Settings/ToolbarButtonStyle/
+<$link to="$:/config/Toolbar/ButtonClass"><<lingo "Hint">></$link>
+
+<$select tiddler="$:/config/Toolbar/ButtonClass">
+<$list filter="[all[shadows+tiddlers]tag[$:/tags/ToolbarButtonStyle]]">
+<option value={{!!text}}>{{!!caption}}</option>
+</$list>
+</$select>