* fix for "Open" tab placeholders in FireFox
this PR fixes the placeholders in FireFox not being removed on drag-leave from time to time
it consists of 2 mods where apparently both are needed:
- creating a `droppable-item` macro where whitespace is trimmed. that macro contains the droppable and inserts the placeholders
- removing the ` ` entity in favor of an inline style `height:2em;` on the placeholder div, putting it in a macro where whitespace can be trimmed, too
I'm investigating if there's a similar fix for the top page dropzone
* remove unnecessary whitespace trim, add css classes
`.tc-sidebar-tab-open-item { position: relative; }`
`.tc-sidebar-tab-open .tc-droppable-placeholer { line-height:2em; height:2em; } `
* replace times entity with close-button
* add sidebar-open-tab styles
77e6f5c0e3